subwoofer question

I'd go with one better sub rather than 2 lesser ones. You can always add a second one later.

The other issue is that the amplifier in the sub is also 15 years old, and you never know when it will need the capacitors changed, etc.

Check the NHT subwoofer close-outs.I bought the A1/X1/W1 system for my HT set-up.It is a very good subwoofer system.My friend bought the same set-up with the single woofer sub enclosures for his bass augmentation ,under the Merlin mxe.He complained a little about the transparency,but this shouldn't be an issue in a HT set-up (.1).He changed the crossover out to an LP-1 and is extremely happy.
The benefit of the X-1/2 crossover in a HT set-up is that there are a myriad of controls at your disposal:X-over freq,phase and boundary control.It also allows for use of two subs and additional routing for a summed/single unit.

Get a newer one, those subwoofer amps don't last forever. I got two (HSU & A/D/S)subs that both amps just stop working after a few years.