Quatre dg-250 amplifier

my dg100 died after only a few years after buying it out the door of the factory in the San Fernando valley. The factory replaced the circuit boards with 250 models, and it has worked
since without any problems. Great bass no hum or vibration as mentioned earlier.

My question is what is the output impedance? I've run it into
B&W DM5's, both as a parallel pairs and singularly- 8 v's 4 ohms. As it is now "long in the tooth", I'm concerned about which configuration is healthier. BTW I also use an APT preamp.

I would be rather careful what you choose to connect to it as some of those amps were not particularly stable with all loads. They liked to blow up a lot. I would probably keep it between 4-8 ohms. No low or complex impedance i.e. no electrostats or ribbons. Resistive as opposed to capacitive.
Wow an Apt Holman Preamp. Make sure the Apt isn't outputting any DC. If I'm not mistaken, that was a AC (capacitor coupled) preamp.

In SD?
Try Stereo Design off of Clairmont Mesa, near where it crosses the 805.

http://stereodesign.com/

Been there 20+ years and have a huge operation.

I'm in L.A. if you are closer.

I used to work at Quatre when they were on Remmet in Chatsworth (or was that Canoga Park?)   The heavy-set guy's name was Guy Hickey. 

If you were running a 8 ohm load the amps were fine.   6 okay.   But if dipped below 4 ohms - I wouldn't use it. 

That said, they had a great sound. The bass was nothing less than awesome - still is.  I'm not sure if you could buy a better bass amp for any money now - but then I've never done a comparison test with anything modern.  I do remember the Dahlquists were a little bass-shy - but the Gain Cell made  world of different.  I was their sales and marketing guy and I advertised the amp as being a better solution than buying the Dahlquist sub.   And it did sound better.   TIGHT. EXCITING BASS.  The Dahlquist sub was kinda tubby-sounding.   I remember someone brought in a pair of AR3a's.   A vintage, but respected speaker at the time.  The bass on the gain cell blew away the owner (and everyone in the shop - I remember getting everyone in the back - the factor - to come and listen. It was a real "wow" moment.)   Sealed cabinet speakers just went crazy for this amp.   But when they went, they went into DC offset - and shot out a nice spurt of DC - which would just push the woofer out and melt the voice coil in place.  Awful.