what exactly is ht bypass

The subwoofer can be a problem, although there a several work around solutions.

The sub needs to stay at the reciever to remain active at it's present level.

You could move it to the new tube preamp...this does involve some amount of trade off...loss of (LFE's).

I would go into detail regarding any advantage/dis-advantage....but I'm not yet clear on your goals, and don't want to confuse you with info overload.

In my system...which is setup for several playback options (audiophile, and non-audiophile), I do the easy way out....run more than one sub.

Dave

Yes, If you can switch between those two inputs...or the sub will do the switching for you.

Just make sure that the manual states that you can use both inputs at the same time...and not, one or the other.

Dave