Newbie Can't Decide Which Tube Amp

@ckenr, with your noted "cramped quarters" and space constraints limited to depth vs. width, you might want to reply with your LxWxH limits.

This might help folks to help you to narrow down a list of possible tube amps(s) you can do given your circumstances. Remember, tube amps need a little space round them for ventilation too.   

@yogiboy 
@hilde45 My friend has 3 year old M120 amps. They can be used with KT88 tubes.

Best to check with Mike @ QS and give him the serial # of the amps, and reconfirm to be sure. The latest version of the Mono 120s with the large transformers in the past two years may have a different recommendation. Not sure if it's plate voltage or what changed, if anything. Or, maybe something was learned since. I ran KT120s and KT150s in my Mono 120 amps, no issues. In fact, originally I tried to order them with KT88s. No-go.  On three separate occasions while checking on internal biasing of the small tubes, I asked and pressed it with him pretty good asking more than once.  Mike told me no, "not" to run KT88s, that I'd, quote: "burn them up" in my Mono 120s.  His web page has been updated since then.  KT88s are removed as an option for output tubes now for the QS Mono 120 amps. Definitely worth re-checking. Best of luck.  

Look here:  
http://quicksilveraudio.com/products/mono-120/  

Mono 120 now shows: "Tube Complement1 12AU7 input, 1 12AT7 driver, 2 KT120 or KT150 outputs. 

@ckenr,
It appears at least 2 different Concerto models existed, each had different drivers in them. The later model has a diffraction device on the tweeter as one indicator, and look closely at the woofers. This original model reviewed way back was rated at 86/db at 8ohms, and yet the other version showed 88db at 6ohms. Old review here, https://www.technologyfactory.com/reviews/SonusFaber/concerto_stereophile.pdf

Agree, its likely going to take a capable amplifier (tube or SS) for these speakers to get proper performance out of them - as your tech and others have already noted. The Tech Factory review above noted these like a clean 100w amp, capable of peaks up to 250w; betting that was a solid state amp too. I like and use tube amps, and in this case, if I kept the Concertos, it would mostly likely be paired with a really nice sounding solid state amp. You could find a number of nice integrated solid state amps in your noted price range. Or, if you must go tube, maybe others speakers as many here suggested.

Is there a label on the back indicating 6 or 8 ohms? Does it have the smaller soft dome tweeter or the version with the diffraction 3-point star diffraction device on the front of the tweeters?