I`ve had a pair of 601 Series IIs in my collection for about 30 years. Purchased them from a Lafayette Radio Electronics store that was about to close it`s doors for good. IMHO, these are speakers that give their best when driven by adequate horsepower....I`ve run them off a Carver M500t, and also a Pioneer SX-1250. They sound smooth, open, and nicely-detailed, with substantial bass energy with either amp.
I normally use Dire Straits` "Brothers In Arms" album as one of my reference sources when auditioning speakers, it covers all the frequency ranges quite nicely. To my ears, the 601s really do it justice.