I believe there was a Shaun Fogg mod for a device once upon a time. I can't fully remember which unit it was but I haven't seen any advertised in a very long time... I'm sure some forum members have used it though.

HD621 is a wonderful device though and very keenly priced these days!

Kind regards
David

G65, 210, HD621, DSP5200.2, DSP3300C, DSP3200,

NUC 7i5 Roon Core, Apple TV 4K, Blustream SW41AB-V2, Panasonic TX55fx802, EE8 switch

Multiroom - Ms200, Nile’s ZR4
Bedroom - 251