Need help picking PreAmp

I was just focusing on a preamp as it seemed like you wanted to keep your amp for now, but an integrated amp is certainly a good option as well if you’re open to selling your Adcom. I’d think the CAP-151 could be a good match, but I’m not familiar with it so not comfortable recommending it. Especially the earlier N805s were known to be very picky about upstream electronics, so I’m hoping some N805 owners might chime in here as they’ll have better hands-on experience regarding what works best.

That said, you could still try the Krell preamp I suggested above as it’ll get you going and likely sound at least very good in your system, and it's good enough to not be out of place if you upgrade your amp down the road. You can always turn around and sell it likely for little loss if/when you get the urge to upgrade.

The only other thing I’d mention is to not ignore the importance of good cables, interconnects, and power cords. If you don’t already have good ones, I’d recommend contacting @grannyring here and have him whip you up some Dueland wires once you get your equipment sorted out. Cullen is making some high value power cords and he sells direct, or you can make your own fairly easily and save a bundle. Sorry if I’m just complicating things here, but just FYI.

A GFP-750 would be a great choice, but it's old and had reliability issues even back in its heyday, so proceed with caution. 

Personally, I wouldn't double down on the GFA-555.  The one you have should provide enough power for the 805s. If you're looking to improve amp performance I'd sell the 555 and upgrade to something better from this millennium (perhaps something from Classe, which is known to pair well with B&W speakers).  But that's me. 

I assume you have the original N805s, and if so you don't want anything that will irritate those metal tweets.  There's a nice Krell KAV-250p available here now for $750 that may fit the bill, and it should pair nicely with your amp.  Best of luck.