Hi there. Recently new owner of a Subway D-800+ here. Probably I don't have the stamina to document this properly right now, but I wanted to mention it. After searching this thread, I didn't quite find any reference to the following:
In the Subway D-800+ preamp, all the parameters together make it an extremely flexible sound shaping tool for bass guitar. And very importantly that includes the variable highpass filter. That's right: the HPF is not only for utility.
Details aside, some weeks ago I discovered that the deep switch on my Kern preamp was actually a fixed shelved EQ that attenuated the lowest frequencies when set off, not boosted them when set on; that this was true at least when the variable bass control was nearly shut. With that setup as a baseline, I was then able to ride the bass control to use it as a "fatness" adjustment. So powerful and wide spectrum. So very bass guitar.
The key there was changing the scale of what low end was within the whole instrument sound. No surprise, since changing scale (of anything; time, frequency, amplitude, etc.) is a favorite sound design technique of mine. But previously that eluded me with the Kern preamp's EQ.
Similarly with the D-800+ preamp, not only does that 'deep on and bass attenuated' technique work well, but also the HPF can be used to further change the scale. For bass guitar, this is an audio illusionist's (me, that is) dream come true. It is very powerful. Carefully throw the voice control into that mix and you can pretend that your bass guitar is musical instrument amplified/miked/monitored/etc. all sorts of ways. Impressive.
Yesterday I got into that with my D-800+. Tonight I got further into it, additionally using the high mid and treble controls for sculpting by attenuation. All I missed was drive, but I have many excellent pedals for that.
The Subway D-800+ variable HPF: much more than a dessert topping.