Tubes on top and Class D on the bottom

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IMO, you can certainly do it, at the expense of musical involvement.

The sound will never be cohesive with different amps on top and bottom.

It can be fun with each amp working in it's "best range", but it won't be cohesive or ultimately satisfying.
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this reminds me of a dirty joke involving a pre-op. no wait, that was boobes on top and...

Tgrisham, I think it all depends on the power output. Now for an amp that simply goes below 20Hz... well, most of them do. But to play 20Hz properly you usually need to go lower than that. There are many tube amps that go to 10Hz. The thing that really helps a tube amp to play bass though is simply to make sure the load at low frequencies is not difficult to drive. That makes all the difference in the world.

Thanks to all....I continue to learn. I will post my final decision, when it happens. Any and all info is appreciated.

Tom