looking for an integrated amp in the $1500 range

Probably a step down in a few ways, and you'd be buying an internal DAC and Phono Stage, that you already have, but..

The Parasound NewClassic 200 Integrated is a nice class D Integrated.

What makes it great for low-level listening is it's feature-ridden, back-lit, remote. From the remote you can adjust bass, treble and sub levels, turn sub off/on (great for placement testing) and you can return to flat levels at the touch of one button. I've driven B&W 606 s2 Anniversary editions and Kef R30's with great results..at normal and low levels.

Much of my listening is at low level and I've really enjoyed this little integrated. I had separates before and downsizing pushed me to look at Integrated. No regrets! 

They run in the $1,200 to $1,300 range, new.

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.

I found that low level listening was best done with super quiet gear. I had a lot of success with Benchmark AHB2 and LA4 preamp for that type of listening. Music details at low volume are not obscured by noisy gear.

Now if you want to skip the low level requirement but in the same physical space. Get a RAAL SR1a earphone and have 2 channel sound at any volume level in your space. I listen to the SR1a or CA-1a (not 2-channel sound) late at night, such as right now, when everyone is sleeping. Listening to Led Zeppelin now and it is as loud as I want it. I am using a Schitt Aegir amp for this ($800).

Look at Musical fidelity M3i  ,  very good amplifier.

Try an Allen Eaton tube amp (no need for pre), and a pair of caintuck audios fast 15 - that setup is slightly over budget but man will it sound magical at low to mid volume levels. Great combo.