All-in-one integrated, 10-15K range, class A/B only

The gryphon Diablo 300 is built world class looks great and beats 
seperates that cost over $20 k .my brother owns one .just check out any current review ,for less then $8k the New Krell 300 is very good sounding and has hdmi,inputs and outputs meaning you can have a direct feed to your  HD TV, both have a nice warmer natural balance.the Gryphon is the best integrated I have heard to date and I have heard the $20k Dartzeel which is excellent the gryphon 
is more powerful and will double down to under 2 ohms to over 900wpc  great head room which you should always have much more then you need. For sure check ot the Krell though 
it stays in pure class A up to over 90 wpc Thatis more then most people listen to at90+ db spl. You didnot mention what kind of speakers you are driving I owned a Audio store for years and system synergy is very important ,when looking to upgrade 
please state your speakers  and efficiency .

If you go the route of using a separate dac/ streamer, try the Naims. Even the entry level model has great sound. 

The Hegel H590 is 300wpc and $11k new. Open box around $8k. Reviews are very good. The H390 seems to be the better deal at 250wpc and the same dac and features. It is $6k, but it can be purchased closer to $5k. I recently upgraded from a NAD C388 to the H390 and I absolutely love the H390!

I own an Aesthetix Mimas with DAC and phono modules. Excellent integrated and under $10k. 

The reviews all said the 390 was great for its price but that the 590 was on a different level, and the 590 is well within the budget, so might as well get the best for the money willing to spend. If wattage was the only difference and didn’t feel the need for then that’s different. Having said that it sounds like speakers arnt set yet and I do believe I saw Maggie’s 20.7 mentioned as a possibility( they can suck up lots more power than 300 watts no problem) And also that he was looking to go with efficient speakers so much this time. Based on that, I’d look into as much power as possible, and for anyone who bass is a big deal, the more power, the better the control over the bass.