amp or preamp to extend upper end

The MF stuff is clearly not doing everything you're looking for due to how it's voiced, so I'd dump both the pre and amp and start from scratch. Trying to mix n match to try to compensate seems like a potential exercise in frustration. Square peg round hole thing. From my experience Bryston gear does what you're looking for, or maybe go back to Krell. Best of luck.

"...BMW always drives. One of the most perfectly engineered automobile and easiest in DIY maintenance." Not sure about that last part. Witness my neighbor who just got stranded on the NJ Turnpike in his 328. Or another neighbor who just spent $5k tracking down an electrical problem in his X5. Electronics, electronics, electronics. The bane of most Euro cars, and uber expensive to fix. Love Bimmers and they're certainly better than most of the breed, but I think your statement pertains more to a 60s/70s VW Bug than a late model BMW.

"I'm confident on all models upto 2007. Parts are not overkill and work on any repair is faster than in any Toyota or Honda.
Expensive maintenance prices only for the fact having european luxury car."

Parts are not overkill??? You must be joking. You must live in Germany or somewhere in Europe. Here in the US they are absolutely absurd. A Euro luxury car shouldn't have to be synonymous with frequent electronic failures and absurdly expensive repairs. If European luxury continues to equal frequent expensive maintenance their allure will eventually wane as others close the gap. And this from a big fan of German cars.

Until they don't. Therein lies the rub.