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. 

The Marantz PM8006 is a nice sounding piece in your budget. It  also had an exceptionally good sounding built in phono stage for your budget (from what i remember), which may make you sell the Rega Booga phono piece you've got...one less pestilent cable, power supply, etc to deal with in the chain.

Thanks perkadin, Unfortunately, I won't have room for floorstanding speakers

Certainly to consider one les component and cables. Thanks

The Quads (all versions) can't be beat for low-level resolution. Regardless of whether made in the UK or China.