Preamp or int amp best way to imprv HT 2 chl SQ?

I've gone through quite an upgrade of my HT system. It now includes B&W N803 mains, HTM1, DS7, Sunfire sub, Sunfire TGA5200 amp (5x200/400wpc 4/8ohm), Sunfire TGP5 pre/pro, modded Denon 3930ci. Everything sounds great, but I'm wondering if I can still improve 2 channel SQ, and how would I best implement that within my budget (3-4k USED?) and size limitations.

I am considering a preamp with HT bypass, possibly adding a 2 channel amp for the mains later(or a 5 channel amp to replace the Sunfire) or an integrated to do both. I can't fit monster amp or integrated amp, so I need a relatively low profile/low heat amp, so I'm considering a Nuforce or maybe Spectron amp. On the preamp side, I'm thinking Red Wine Isabella (he'll add HT bypass), Modwright 9.0 or 36.5, or Dodd (Aesthetix is possibility, but it seems to have noise problems). Integrated, thinking about Mimetism, Rowland 500, etc, but not sure I can fit it and these are more expensive, but I've heard they are pretty great.

So, besides your thoughts on the pieces I've selected, which is my best upgrade path in your opinion? Will a good preamp make the biggest difference with this setup and amp, since many believe pre/pro is weak in 2 channel versus any good preamp? Should I upgrade amp instead? Or would a good integrated do the trick? I assume for 3-5k used I can get a pretty good integrated, no? Look forward to your opinions.