Theil 3.5's...Tubes or SS? Just bought a used pair

I have owned a pair of Thiel 3.5s since 1996 - (bought them on consignment from the original owner) - and have used solid state amps since I first bought them. I started out with an Adcom GFA-555II amp rated at 200 WPC and later moved on to an all McIntosh setup - that is driven with an MC-150 amp - which is rated at 150WPC.
Though the McIntosh amp has a lower WPC rating it can easily drive loads in the 3-4 Ohm range. The tube amps that Marakanetz lists above are all good choices if you really choose to go that route. If you decide to go with a SS setup, Thiel 3.5s are well matched with amps made by Conrad Johnson and McIntosh. Both sound very smooth yet detailed.
I would personally not choose a SS amp rated less than 100 WPC to drive the Thiels. They do tend to sound better with more power; even in a small room. And by all means, don't forget to hook your 3.5s up with decent speaker cables.
One last thing, since you don't have a manual for your speakers - make a point of calling up Thiel and asking them to send you a copy. They are super-helpful and will get one off to you for little or no cost. The oners manual is short, but will offer some very good tips for placement and getting the best sound out of your speakers.
If you need any other suggestions for your 3.5s, feel free to send me an e-mail.

The 3.5's are 89db sensitivity.
I drive mine with an Electrocompaniet AW-75
Its 75wpc/8ohm 145/4ohm and ~200/2ohm
Its has 30 amps per side and peaks at ~80 amps.
This amp and 3.5's make a great combo.The AW-75 sounds like tubes with balls.

Your amp requirements will depend on your room, desired listening levels, system, prefered music and individual taste. With that in mind, I suggest you find something that can deliver at least 200 Watts into 4 Ohms. The 3.5's are not forgiving of abberations in the upper frequency range and benefit from an amp that can deal with the demands the equalizer presents in the lower frequenicies. With all due respect to Rcprince the best amps I've heard on the 3.5's were Krell KMA 160's in an all Krell system. I have not heard them with the Thresholds that were rumoured to be used in their development. I did hear them sound nice but underpowered with Forte, so I think it's fair to presume that the higher powered (quality?) Thresholds might work well. I have heard them sound nice with big solid state c-j's (A's) and would imagine that the right Pass equipment would work well. I did not care for the sound with Adcom, Rotel, Mark Levinson, Spectral, McCormack or c-j tubes, and all for different reasons. I can't speak from experience but Audio Research tubes are rumoured to work well. Pertinent 3.5' specs: Bandwidth (-3dB) 20Hz-22KHz, Amplitude response 23HZ-20KHz +/-2 dB, Sensitivity 88/89dB (depending on vintage) @ 1 watt-meter, Impedance 4 Ohms (both nominal and minimum), Recommended Power 40/50 (depending on vintage) - 250 watts. When I questioned Jim Thiel about the power recommendations he said that they were used with the standard 8 Ohm ratings of quality solid state amps that were able to "double down" in mind, and suggested doubling those figures if using tubes. IMHO the 3.5's (and most Thiels) cry out for high quality amplification to make them sing. Good luck!

P.S. Rowlands work nicely too.

Actually, Unsound, no offense taken--I would not doubt the Krell KMA 160s would sound great on the Thiels, as I was always fond of that generation of Krells, but not one of the first production run, just taken new out of the box and plugged in for 5 minutes. :^) Once my KSA 80 was broken in and when it was warmed up I liked it quite a bit, it was very sweet in the highs, and indeed would make a nice match; in fact, prior to that I had used a KSA 50 Mk II on my Thiel CS3s with very nice results.