IMHO:...,
Your room might be too small for theses speakers. With their minimal baffles and also inherent warm, ripe sonice signature; they need space from surrounding walls. They also need space from the listening position for proper driver intergration.
Unless your seeking very high output levels, (which aren’t these speakers forte’) the Threshold S 150 should work well. The Series II versions were a nice upgrade,. later ones with the pre-Optical Bias version corrosive resistent gold plated circuit boards are worth seeking out, especially considering the age of these amps. As has been previsouly posted the original caps need replacing. Jan Soderberg an ex-Threshold tech at Vintage Amp Repair has earned an excellent reputation for updating Threshold gear. Unlike other modders whose handi-work can actually devalue gear, his work will be apprecaited and sought out. Finding a piece that has fairly recently gone through his hands might be the most long term cost effective route.
While I can’t disagree with @ghdprentice ’s perception, I think it important to remember that it is a perception. If you look through the posts here on Audiogon you’ll find some prefer the Threshold to the Pass Labs amps and visa versa. I would suggest that these earlier Thresholds are rather neutral with a very slight leaning to warmth, where as the Pass Labs are most certainly in the warmth camp. As the Vandersteen are already strongly leaning towards the warm, ripe spectrum, it might be advisable not to accentuate that with an overly warm amp.
YMMV
Amongst my favoriite amps for these Vandy’s are the McCormack’s and the PSE’s. Regretably they didn’t have the robust build quality of amps like these older Thresholds and are no longer an easy reccomendation.