Are there alternatives to an NHT X1?

You have 2 passive NHT subwoofers , 1 mono amp and no X1 external crossover? 

Correct. I suspect some components were lost in a move

Only thing I can think of is trying an active crossover like a Minidsp 2x4hd, of course you'll need another amp to use both subwoofers. 

This may be your lucky day!  I have a A1 & X1 I have not used for years.  Message me here.  If we can come to an agreement I'll post them for sale here.

Did you sell your A1 & X1? OP, you need the X1 because it provides the necessary EQ for the NHT subwoofers. The natural -3dB point of the subs is 58Hz but the X1 drops the -3 dB point down to 26Hz. I guess if you had to, you might be able to get by by manually EQing the subs and use an electronic crossover. Stereophile reports that the X1 adds 12dB at 32Hz.

No I did not post it yet.  I did test it and I could not get the crossover to work.  The A1 & X1 have been sitting unused for years and my system has changed many times.  I will give another go later today and get back to you.