I read this from Stan on another forum:
The 7510+ is not a replacement, but an addition in my line up. Not everyone wants USB or the addition of a more expensive headphone amp section, or wants to do opamp rolling for that matter. And fewer still are using their DAC as a digital preamp. So I don't expect it to be selling just as well as the TC-7510.
I see the 7510+ as a specialized product that would be more of interest to people who want something close to Chord64 like performance at less than £200. The TC-7510 is more aimed at the high-end CDP substitution market, as many a Naim, Linn, and Wadia CDP owner who got a TC-7510 can testify to.