I did a 30 minute dig through the links you’ve provided and it seems that I could switch between the pot and a 100k resistor to max the decay time from the IC. If I wasn’t happy with the ‘tone’ of the reverb, I’d probably have to dig into the PT2399 chip, because it seems that the BTDR3H has a set LPF that is part of it’s design. Maybe that’s why the Add-Verb has a tone control when integrating the stereo signals to mono… but you’d always be starting with the output of the last PT2399’s LPF.
If you’re willing to share, I’d be interested in how you added the modulation to the D’Lay board. Fundamentally, it seems like I would be using the same components since the BTDR3H is three PT2399’s.