Scoring on Used Thiels here

Thiel appeals to me.  The low efficiency is a concern for me.

The efficiency is about average, actually. Most models rated 87-89 dB. The "problem" is the low impedance. Thiels draw a lot of current. You will want a high quality amp with a decent 4 ohm rating.

Years ago I drove a pair of CS1.6s with an Ayre AX-7e. That amp is rated at "only" 60 W but doubles into a 4 ohm load. That amp had plenty of balls to drive the Thiels as loudly as I wanted.

The amp quality part is very important. Thiels are highly resolving and will reveal weaknesses in your amp, cables, and sources. I'd MUCH rather have a high quality 60 W amp (again, with a 4 ohm rating) than a 200 W meh amp. BTW, a zero-feedback design sounds fantastic with Thiels, IME.

What Thiel models are you considering?

C5i doubles down to 4 ohms according to specs. I don’t see a problem there. It does well with my Dynaudio Contour 1.3mkii monitors which are likely in same class as Thiel in terms of amps needed for best results. SPL is somewhat limited I would say.

I am open to any model though the less obtrusive physically the better.  WAF matters in that room. 

How much bass can the room handle?

Hi,
I caught your post, because I have a possibility to get ones to restore CS 3.5.  This pair will require at least replacement of one woofer and foam surrounds. I've never been in Thiel camp, but more certain than not that I have proper electronics to push. 
Owner will accept trade and I'm planning to trade Technics SL-D3 fully auto turntable only no headshell, dust cover or cartridge in full operational automatic condition. The turntable like that probably has value of $89max, but certainly was picked up by me from estate junk to restore.

mapman - Your Bel Canto integrated should be fine.  It's rated at 60 watts @ 8 ohms, but double to 120 watts @ 4 ohms...always a good sign of a well-engineered amplifier.  More importantly, it's able to deliver 30 amperes of peak current, so it should be able to handle most any Thiel quite well, even the notoriously demanding 3.6's & 3.7's.

Bel Canto's amplification products typically measure much better than they rate them, so you may be well surprised at how loud you can make the Thiels play.  Again, just be mindful that the woofers, mid-bass & tweeters are handling a wider range of frequencies than usual due to the First-Order crossovers, so blowing a driver is a concern if you ever move on to something with 300+ watts.  At 120, you should be fine... 

What I've found is that, while they instantly captivate & amaze at lower & moderate listening volumes, the Thiels really "clear their throats" as you start going past 12:00 on the volume knob.  They LOVE to sing with some real volume being applied.  It sometimes becomes a bit "uncomfortable" listening this loud, but it's astonishing how easy & detailed the presentation is at these sound levels.  Amazing speakers...

My wife wasn't exactly thrilled when I moved from stand-mount monitors (Martin Logan LX-16's w/Dynamo 300 sub) to the full-range 3.5's, but she appreciated the fact that the wood & cabinetry was much more "high-end", looking more like furniture than electronic equipment. Also, as my Thiels are truly full-range, I was able to ditch the sub as well, which resulted in a much more cohesive sounding system (the Thiels seem to play lower than the ML system ever did with the sub!) One less box in the room made my wife happy.

Please keep us updated as you search for that perfect pair!