Upgrade for old Dahlquist DQ LP1 Crossover

I use Marchand's excellent (IMHO) XM-9 for my stereo subs. It does have a mono summing switch on it, but I have never used that feature. I find that biamping with passive subs and an electronic x-over is a great advantage. I don't hear any degredation in the sound with the Marchand.

Have you seen this link? http://www.regnar.com/cgi-local/SoftCart.100.exe/online-store/scstore/p-DQLP1CRUK.html?L+scstore+rhvk5229ff794b79+1051030992

At one time, Frank Van Alstine modified these units. Cost unknown. You can reach him at 952-890-3517.
It was a Van Alstine modified DQ LP1 that Harry Pearson used in his writings about one of his then listening room system which consisted of Quad ESL63's with Maggy Timpani panels for the low end

This is an old post but I am responding to it as I am still using my LP1.  I did modify mine to it is now singled ended from a +30Vdc supply, rather than the +/- 15Vdc.  That meant putting polypropylene caps on the output to block the DC.   The design did add significant bass impact as it eliminated the sensitivity of the op amp to noise on the negative rail.  

Definitely use polystyrene caps on the upper pass region.