Upgrade for old Dahlquist DQ LP1 Crossover

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.  

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

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

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.

I upgraded the caps and resistors in mine and changed all of the jacks. The most important components in this crossover are the caps used on the passive side. These caps are in the signal path of the main speakers and need to be polyprop or polystyrene. Do not, under any circumstances, use the dipped silver mica caps that come with the unit. They sound terrible.

At one time when I was into the subwoofer thing I was using a LP1 modified by a local technician who upgraded all the capacitors and resistors. The improvement was clearly audible with less veiling and more transparity across the frequency spectrum. I sold the unit to a friend who used it for awhile until a speaker change put it in the closet where it still resides to this day.

So, if you're thinking of mod-ing it, by all means, do so. The results were worth the cost for me.

Are there any mods or tweaks that could be done to the DQ LP1 if I decide to keep it?

They still bring top dollar today. Very desirable due to it's flexability. I would keep it.