whats the best pre amp to pair with Bryston 4b cubed? A

yes well if the bryston amp is not a done deal, my no-brainer suggestion is get a hegel h190 and be done... superb amp, sweet combo with totems

The Arcam is a nice amp. I'm not convinced it's going to make a huge jump.

I tried a set of 7b3 monoblocks in my system at one point in time.  It turned out to be pretty good after an internal fuse upgrade, but the overall sound was thin in bass, midbass, midrange.  I would pair this with a very thick sounding preamp with heavy on the midrange texture and warm slow bass.

This is my experience also.I did not work together with audio research ref3 preamp

Thin and bright sound.Things improved with onkyo p3000r preamp

The cubed series seems to have thickened the house sound of the Bryston line to add more tone and heft to the sound versus earlier versions.  I will just say this — I bought a Bryston BP6 because it was supremely transparent and provided the most unvarnished and clear, 3D soundstage I’d ever heard in my system, by far.  If you’re looking for no coloration and an expansive 3D soundstage, go for a used BP26 and you’ll be thrilled.  If you’re looking for more flesh on the bones, go for a VAC or deHavilland kinda thing with tubes if you’re not opposed to that.  Just depends what sound characteristics you’re after.  But, if soundstage and transparency are your primary goals, get a BP26 and you’ll be thrilled. 

Have you looked or listened to other high end brands besides Bryston?