I'll try to make this as short as I can:

I play music that has a lot looping and uses just me on guitar. So a lot of times I like to play bass by using a whammy and dropping everything an octave on a 7-string guitar. Playing bass through a guitar cab isn't necessarily the best and bad for the speakers in general.

I like how the guitar sounds through a bass cab just fine and I've been using one. My question is: I'd like switch to a tube amp for the bass cab. Good idea or no?

It's a 400 watt swr workingpro.

Second question: Which amp and what wattage?
A detuned guitar doesn't kill speakers like an actual bass guitar. It isn't the frequency that does the damage, it's the attack.
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I played guitar through a Hartke 410 for a while. I liked the sound it gave me, really strange metal tone, but I liked it.

and +1 to AndyGray.

If you're not playing too loud it shouldn't be that bad for the speakers. Playing shows or practices really loud might cause some speaker damage. I've messed around with the 1 octave down on my Vetta a couple times and if you're playing loud the speakers move quite a bit more than I'm comfortable with... think subs in a car playing dub step.
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