The Rega speakers are very good choice for guitar oriented music, if you are of the mind a guitar should actually sound like a guitar. They came out of England and were designed for a company dedicated to producing affordable yet "high end" audio products. Rega is best known for their turntables which have been around since the 1970's and have always been considered excellent high value products. Their CD players are among the best in their price range and twice that in most cases. That's the basic idea Rega sells, a lot of music for not a lot of money.
Coming from England, the Rega line of speakers tended towards what would have been termed "the BBC balance" which places a strong emphasis on the midrange and vocal qualities of reproduced music. Probably, the most well known of the British speakers would have been the landmark LS3/5a (https://www.google.com/search?q=ls3%2F5 ... e&ie=UTF-8
) which was designed by the BBC to be used in mobile monitoring situations. Quad is another well known British speaker (https://www.google.com/search?rlz=1CAAC ... uwu5CVbjeQ
) which has survived as a true legend in audio since 1957. You can expect many of the best qualities represented in these loudspeakers to be present in the Rega's. If you've ever heard American speakers divided into East Coast sound and West Coast sound, the Rega's lean more towards the East Coast sound, more a classical music speaker than a rock and roll speaker. Bass is somewhat tame while taut and mids are expressive. Think of a good cup of English Breakfast tea vs a cup o' Folgers.
The BBC sound could be described as generically flat/neutral frequency response with a fidelity to the source of reproduced music. And that's where your question becomes less simple and more difficult to answer. British speakers designed around the BBC ideal tend to have strengths which the average listener would likely miss, if not dismiss, as not being important. For the listener intimately familiar with the qualities of live music, however, nailing the musical flow and momentum are often of prime concern. Either placing the performer within your room or you within the original performance venue are talents of the British designers you should find evident in the Rega's. The Rega's score high on the points related to what would you have heard at the original performance. Dynamics are somewhat limited in any small loudspeaker due to the available excursion of the low frequency driver's voice coil. While the small bookshelf Rega probably wouldn't be your choice for live performance level reproduction of a full symphony, they should do nicely for guitar music and with judicious use of the volume control will do well with any sort of live music source.
Two things you should realize about high end loudspeakers which are, IMO, somewhat critical to achieving best performance from any high end loudspeaker, but particularly British speakers, are; first, the speaker is the least important part of the system as it only puts out what has been put into it and, secondly, the room is as much as 90% of what you perceive from any audio system.
The first rule goes to the computer speak concept of "garbage in = garbage out". In other words, what you feed the speaker in terms of quality in the system in front of the speaker, particularly the source player, will be what you get out of the speaker. Feed the Rega a crummy signal from a cheap source player or a poorly designed amplifier and you will hear the results of where corners have been cut in front of the Rega. Therefore, you don't need to spend big money on electronics, cables and such but if you want to drive the Regas with a cheap mass market AVR receiver, that's what the Rega's will show you in terms of musical flow and less tangible values such as "presentation". Given the right equipment in front of them, the only real draw back to the small Rega's is their extension in the lowest octave though they should be more than adequate for the range of a six string guitar. Otherwise, a lower cost Rega sounds virtually identical to a larger, more expensive Rega. Matching the "personality" of the various pieces is a good idea with a speaker like the Rega. So you might want to consider auditioning some other British electronics to go with your new speakers.
In terms of the room, it begins with the physical interface between the loudspeaker cabinet and the stand on which they sit. Do not cheap out and just plunk the Rega's down where they fit, it will no doubt be the worst location you might have chosen. Read a bit about stands before you expect too much from the Rega's. Stands like the Sanus BF31B are better than placing the speakers on or in a cabinet but they are lightweight and will not fully support the speaker in terms of fidelity. Stands like the Bowers & Wilkins - STAV 24 S2 are heavier and provide more mass to lock the speaker in space. They will be even better if you can fill the tubes with sand. Buying the right stand will improve the performance of your speakers more than you may realize at first but the benefits will become obvious after you have accustomed your perceptions to the stability a stand can provide.
Then read about how to properly set up and place your speakers within a listening room; https://www.google.com/search?q=loudspe ... e&ie=UTF-8
Ideally, a listening room is dedicated to the audio system alone but most domestic rooms can suffice. Often with a domestic room the speakers can be moved in or out of position to enjoy music with very little disruption to the flow of traffic.
Spend a bit on cables for your new system. Not a lot but a little. And most definitely connect your speakers "in phase".
The speaker has relatively high electrical sensitivity (90dB @ 1 watt) and a non-complex impedance load. A well designed and well built amplifier of about 20-30 watts should fill a medium sized room with the realistic presence of a live performance. When buying an amplifier, remember that doubling the wattage ONLY gains you about +3dB of volume potential. The "potential" resides at the top of the volume peaks meaning transient attacks will be ever so slightly cleaner. Otherwise, most listeners will average less than five watts RMS for most music and quite often less than 2 watts are needed with an efficient speaker like the Rega.
Depending on your own musical values, the Rega speakers, when set up well and fed a high quality signal, can prove to be a good sized bite of the high end at a very reasonable cost. Without asking more questions and listening to your specific desires, that's the best I can do for now. If you have any questions, please follow up.