preamplifier after preamplifier?

OP,

In both of your diagrams above you are still putting all of the bass freqs into your main speakers.  Unless you have dual binding posts on your speakers and even then you would be taking out the woofers in the mains entirely.

The passive volume seems a nice work-a-round but see paragraph above.

I have to agree with @daledeee1 that a two-way Marchand electronic cross-over
is the way to go.  They are all custom made in the respect that you chose the 
slope and the cross-over freq.  That way you're not beating up your amp with all
the low bass that the subs are going to play anyway.

If there's more to the story please let us know.

I have no affiliation with Marchand. Just take a peek at their offerings.

www.marchandelec.com

Regards,
barts

I forgot to mention, or ask a question.  The first preamp has gain.  Feeding that to the second preamp, is that too much gain in the end?

Two preamps, no, simply no.

@fiesta75 

+1

I use the high level inputs on my Rel S5 Sho subs.  I have them connected directly to my amp outputs.  Works great.  I believe it is actually sonically better.