looking for an integrated amp in the $1500 range

I don’t think a new amp will help much at lower volumes. Some speakers work at low levels while others suffer. In my experience, high sensitivity designs tend to do much better at low volume. Working within your price range I’d consider the Klipsch Heresy IV used (HifiHeaven). If you sold your amps and speakers you’d be able to build a system around the H4’s and a modest integrated (you wont need much power so you can save budget with the 99db efficient Heresy). $1500 plus another $1200 or so from the S400 sale, would leave you about $500 for an integrated (Arcam, Creek, NAD, Roksan, Rotel, Rega, Yamaha etc.) after you get the Hersey.  The H4 and Rega Brio for example since you are familar w Rega should be attainable. 

I totally understand where you are coming from, selling can be a hassle. But the truth is low level listening has much more to do with the speaker, its impedance curve and its sensitivity, then the amplifier’s characteristics. You already have a very beefy amp which is more then capable of driving your speakers, so if that amp doesn’t do it for you at low volumes, a $1,500 integrated will likely be a downgrade. Plus as I mentioned, high sensitivity speakers may cost a bit more up front, but you can easily get away with a lower cost integrated.

True high sensitivity designs tend to be large to compensate for the bass tradeoff, the Heresy is among the smallest that I know of with a true high 90’s sensitivity. Honestly from a floor space perspective it doesn’t take up much more room then a stand mounted speaker. Are you placing these on a cabinet or shelf? If not I’d seriously consider them, they are awesome. Many people actually place them on stands. I’ve even been to a bar that had some old versions wall mounted. The sound was incredible. I have the Forte IV’s myself and they’ll never leave.

Another way to achieve high sensitivity without a huge box is to get rid of the crossover and use a full range driver. Zu Audio has a few designs like this and they do make a bookshelf, though its a rather large bookshelf. Check out the Zu DWX. I have no personal experience with the DWX though. Others may be able to chime in.