So I spent considerable time doing the demo with the Momentum HD preamp. I always said I would never sell my Ref40 (I hear you laughing now, why would any audiophile say 'I would never buy another piece of equipment!), however it really is a massive step up in terms of performance. Infact mine is due to arrive in the next 2 weeks.
I can only imagine what the new Relentless preamp would be like - I hear it is a 3 box set-up. With the level of your system and amplification this is probably the way to go, however the baby brother which I have is a serious piece of kit. Maybe see if your dealer can lend you one whilst you wait for the repair of the Ref40 / release of new preamp. At least you will be able to ascertain the differences.
Btw I also have a full DCS Vivaldi stack, running Magico S7's with full Transparent Reference / Opus cabling
M.