They're known for being imaging champs. They seem to partner well with tube gear.
The last time I spent any serious time with them was quite some years ago when they were riding their Response series into the pages of Stereophile's reviews.

Read this review that CG did way back when.... http://www.stereophile.com/standloudspeakers/792proac/

I currently have the Vandersteen Treo speakers, and think they sound better than the ProAc speakers. be sure to check them out if you can.

I am using the ARC LS26 preamp (current model is the LS27) and a Musical Fidelity M6PRX power amplifier; sound to die for...lol. The LS26/27 has 7 inputs that are all balanced and unbalanced, more than the DS20 has.

I think that the Reference 75 may be less power than you want for either the ProAcs or the Treos; the Treos certainly need at least 150 watts. The Reference 150 might be a better choice.

I am in love with the Musical Fidelity M6PRX, which puts out over 300 watts per channel and is under $4000. It also is not as big as the tube amps and does not put out the tremendous amount of heat that they do. Its sound quality is very comparable to the ARC amps IMO.

I don't know where you are located, but I highly recommend Randy at Optimal Enchantment in Santa Monica. He has been a leading dealer of both Vandersteen and ARC for many years.