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

I'll agree with Jameswei to some extent, with a caveat and an observation. The 3.5s are still good sounding speakers, and can use a good powerful amp to drive them, but their tonal balance is tilted towards the treble and thus care should be taken to choose an amp that's not going to come across as too "hot" on top if you go with solid state. When I bought my Krell KSA 80 years ago, I listened to it (admittedly not yet broken in) on a pair of 3.5s to get an idea of its sound (I was getting the amp for my Duntechs, which this dealer did not carry, and the Thiels were the only speaker he had with which I was familiar, having previously owned Thiel CS3s). The sound was OK, and sufficed for my purposes of being sure the 80 had enough power and bass control for me, but not broken in the Krell sounded quite hard and bright on the top end. As I was leaving the store, they had just hooked up a Conrad Johnson Premiere 3 (I think it was the 3, it was their top of the line then) to the Thiels for another customer, and I had to stop and come back to listen for the sheer enjoyment of it when I heard that combination. Yes, the bass wasn't nearly as controlled, and perhaps the highs were rolled off, but what a great midrange and soundstage! My first exposure to tubes, which within two years replaced my solid state equipment. So don't be too quick to write off tubed equipment on the Thiels, just make sure it has enough power to control the speaker. A final point, there are some who believe that the 3.5's equalizer to boost its bass is the weak point of the design (there were mods available from JPS, I think, among others, to improve the equalizer); if you don't listen to a ton of organ music you might want to experiment with the EQ in and out of the system to see if the added transparency without the EQ is worth the tradeoff in the last octave of bass extension. Good luck, I think you'll enjoy your time with the Thiels!

As I recall Thiel was using Threshold ss amps to voice their speakers when these came out. I used a Class A Threshold on mine for several years. Sounded better than others I tried at the time. I never tried tubes, but I would imagine if you did you be talking about large powerful PP ones and they might just sound pretty good if they could handle the bass. I use just such amps with my present dynamic speakers to my great delight.

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!