Minimum strength to drive Spendor 3/5

a push/pull 35 and up tube or a 75 and up ss.

Having owned two pairs of S3/5s, I'd have to say it depends, esp. w/ tubes. A 50 W Jolida was only ok, a 50 W Antique Sound amp better, a 27 W (triode) VAC amp much better. The 30/40 W Jolida and ASL integrateds clearly didn't have enough power. I heard them w/ a lot of SS integrated amps; none below the top of the line Creek or the Roksan Caspian really did them justice. The S3/5s will sound dark and "slow" when underpowered, but will open up quite a bit with sufficient power.

Quad amps match with the 3/5's very well.

I personally drive my Chartwell LS3/5a with a Quad 606II and 66 combo.

To get a better idea you should read the review of these speakers which would give you an idea of which amp to drive these with.

http://www.qsandd.com/reviews/s35review.htm