Mati...does your 35wpc tube amp have a 4ohm tap in the back? Thiel recommends 50-150wpc but Sam Tellig, in his review of the speaker (you can find it in Stereophile's archives), ran it with a 20wpc tube amp with decent results. But his tube amp had 4ohm taps in the back which matched well with the speaker because it's nominally a 4ohm impedance speaker, never dipping below 3ohms or going above 5ohms across the frequency spectrum. So that was a good match. I run Thiel 2.4s (so believe me I like the Thiel sound) with ARC Ref 110 (100wpc) in a moderately sized listening room at moderate sound levels and it's enough to drive the speakers. I just don't know about your tube amp. If it has a 4ohm tap and has solid damping factor (allowing it to drive the woofer with control and no mushiness in the bass), then you should be ok.
