Hi Ed,
Please don't think that I think my response to your query is conclusive. I hope it gives more info to help you make a more informed decision about your speaker purchase. I'm sure others will take on your question in a slightly different way or give you a straight recommendation. I can't do that as I've not listened to much Arcam gear or ever heard the Solo products.
Speaker sensitivity (sometimes also called efficiency) is expressed as a figure in decibels (decibels are the unit measure of sound pressure, and is written as "dbs"). The rating is usually determined through a test in controlled conditions. The dbs rating is typically based on what sound level is measured 1M away from the speaker with 1 watt of power. Impedance/resistance (measured in ohm) is also a factor, but for simplicity's sake I will just say that you should stick with 8ohm designs.
A good general rule is that that small speakers will have lower sensitivity than larger speakers, and will therefore need more amp power to create the same/similar sound pressure. Take a small speaker like a Wharfdale Diamond 9.1, or a Quad 11L2, and you are looking at 85dbs to 87dbs. Take a big floorstander and you're more likely to get something in the range of 88 - 92dbs.
If you find a loudspeaker with a 15 inch woofer, it is likely that it will be highly sensitive. If you look at a standmount with a 4 inch mid/woofer, then it is likely to be insensitive. However, if you look at a floorstanding speaker with multiple 4 inchers, then the sensitivity is likely to go up, and the additional woofers help make the loudspeaker easier to drive.
I would say that if you have a small room (less than 4X4M), the vast majority of good standmount hi-fi speakers available today will be perfectly usable with your Solo Mini. I think it's fair to say that anything rated from 86dbs up will be OK for average/low level listening in small rooms. If your room is larger and/or you like it loud and bassy, then I seriously suggest you look for a floorstanding model with 89dbs + sensitivity, or a more unusual high-efficiency design standmount.
I am deliberately simplifying here, but a good thing to keep in mind is that heavy rock, dance and anything with a heavy bass will require more of your system. If you like to listen loud and/or highly price bass, then I think you should consider using higher sensitivity speakers, or add a subwoofer to your system.
Tannoy, Monitor Audio and Acoustic Energy are British brands I associate with better than average sensitivity. Tannoy actually have some super-high efficiency models, but definitely towards the "funny money" end of the spectrum!
French brand Focal make classy, technically brilliant and very efficient loudspeakers. They do have a tendency to overstate their sensitivity ratings though (according to several tests I've read).
The American brand Klipsch make very sensitive loudspeakers. For full disclosure, I am a long-term Klipsch fan and I own the RB-81's that are accurately rated at 97dbs... they go LOUD with 5 watts of valve amp (and I have them connected to 120 watt Roksan!). The trade-off with Klipsch is that their horn tweeters *can* sound biting and bright in the wrong room/system, and
they look bloody ugly!
Final and ultimate word of advice - a home trial is the best way to find out if a speaker
is going to work for you. Second best would be to take along your Solo unit to a dealer to try in their demo room. Good luck, and enjoy the music!
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