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.
Replace the Sunfire with a used Anthem D2 with ARC.

Thanks Kal. That's another good suggestion. I know you are a big fan of room correction, and rightfully so I'm sure. But are you saying that room correction with the D2's analogue section will trump any of the options/equipment I've mentioned above? Guess I would have to re-read your reviews, but I thought that, in your efforts to find the best multi-channel audio, you were a bigger fan of "better" sounding units like the Cary Cinema 11, the Bryston sp2 and the BelCanto Pre6, among others. Anyway, looking forward to more suggestions/opinions on my choices.
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I have to say that the SQ out of my sunfire components is excellent. No question. But I would bet that I can improve further. However, right now I have a limited budget, so I'm looking for opinions on my various upgrade path choices as in my OP. I'm sure that many of you have gone through similar incremental upgrades. Given my existing components, what would make the biggest improvement: amp? preamp?
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Thanks Eric, that's kinda what I'm thinking. May go for a good used preamp like the Modwright 9.0 or 36.5, or the Dodd or Red Wine. Aesthetix has gotten great reviews, but seen on several boards a problem with noise, etc. I would love to hear more opinions though. Anyone...?

Was hoping I'd get a decent amount of responses, but the various boards I'm on, avs, agon, htguide, seem slow these days (at least for my questions,lol).

One other point for Kal's earlier response: rather than get the D2 with ARC, why not spend less money and get an outboard audessey or tact room correction system, and then go for a great preamp/amp? Regardless how good the room correction is, I'm still supposedly limited by the analogue section of the pre/pro (subscribing to the theory that pre/pros can't compete with dedicated 2 channel preamps).
The drawback to the external EQ is, precisely, the additional A/D/A conversions which confound any attempt to maintain a coherent analog signal path. Of course, that is just as much "theory" as the presumption that pre/pros can't compete with dedicated 2 channel preamps. In both cases, there is truth and untruth. It depends.

No doubt Kal. As much as I'd like to get a consensus, I do know that it's all in MY ears ultimately. And though sometimes you get a consensus and sometimes you don't, even a consensus allows for dissidents. If you look over at Zissou's thread at AVS, you see that the consensus is that the Proceed AVP2 pre/pro has about as good an analogue section as most good reasonably priced preamps. However, even so, there are those that bought the Proceed and wound up not liking it as much as they thought, and opted for a 2 channel preamp. In that thread, many claimed, as did Zissou, that the Bryston SP2 was not very good. Yet you and many other opinions I found on the web thought it was an outstanding pre/pro as respects analogue 2 channel playback. I would bet it is. Perhaps the D2 to a preamp is a better bet? Who knows. Guess I will have to try a few options to see how I fare.