Dear Andy A,
Yes, it is "Possible" to mount a DCC decoder under the layout,
patch it's audio outs to a Sub-woofer,
and have it "respond" to the same DCC control commands as the loco "topside".
(Simply add the "Sub woofer" decoder to the "Locomotive consist", as if it was "double heading" with the actual loco).
However, this does not address 2 key issues.
1 - SYNC
If you are using "auto-sync" or "sync to speed step",
AND the sound files/CV config betwen the 2 decoders is Exactly the same,
then they <Should> appear to "chuff" in sync.
If you however are using any form of actual "motor feedback" or "physical cam" chuff sync, the "sub woofer" decoder cannot access these signals, and thus will NOT be in Sync with the loco "chuffs"
2 - Compatibility
What if Loco #1,
which is working fine with the "sub woofer decoder",
(Right sound, CV settings match, sync is behaving, etc etc etc)
has a "hiccup",
and following the old exhibitor's mantra,
you Do NOt Hesitate,
simply swap locos,
and "keep the show going",
(we'll deal with whatever loco #1's issues are later...),
and "Loco #2"'s sound decoder config does NOT match the "sub-woofer decoder"'s config?
(Or worse, the "sub-woofer" is a steam decoder, a
nd you have had to revert to your "fallback" diesel loco?)
Sure, you could have 2 (TWO) decoders mounted under the layout,
(One steam, one diesel),
feeding a 2-into-1 "buffer"/mixer circuit,
and from there into the sub-woofer,
and remember to "add to consist" the appropriate sub-woofer decoder to match the loco currently "onstage",
although in practise, in the heat of exhibition operation battle,
adding these few "extra steps" may make the difference between ENJOYING the weekend at the show,
and tearing you hair out as Again you've run your Lister out on stage, with sub-woofer steam "Chuffs" coming from under the layout...?!?!?
The extra impact of "sub-woofer sound re-inforcement" would be impressive, esp if it is subtle,
(The average viewer won't be able to tell exactly WHAT you've done,
but they will KNOW that "this layout sounds more impressive/REAL than the other ones"),
BUT if it makes life harder on the operators,
then maybe the benefit isn't worth the mutiny...
(I trust, Respect, and Value my operators,
without them I simply couldn't survive a tough exhibiton...)