Any 130V zener will do, any wattage and tolerance will do. I should have some LSK389's but I'd have to check. Just shoot me a PM so I won't forget.
Now it's time to patch up this nasty cut I got while drilling the front panel for the newest amp...
Bdent is just more expensive but the part is out of production so it's not going to get any cheaper. No sub available with the same or better specs.
I can help you with 2705's as I have a bag full of them and any 130V zener will work. I got some from Mouser recently but don't remember the partnumber...
Get the "cheap" MSB dac for a nice compliment to the BHSE. The USB inputs are not cheap (even my dealer complains about his price) but the best I've ever tried.
Relays and a stepped switch would be an easy way to adjust the gain. Now if you want it to be a volume control then you are looking at KX-R levels of complexity though...