What does an impedance-matching issue "sound" like?

I've never been a fan of anything paralleled or bridged. IMO, the sonics always suffer.

Looking at the mq88 schematic, there is a complex output filter and a multitude of pots for bias, gain and balance. It is possible that the particular combination of all the parts moves the channel symmetry enough at certain frequencies to cause an imbalance. Tube non-linearities/sub-par tube could also contribute.

Transformers like to see the load for which they are designed. The best advice here is to Bi-Amp. Doing so could help to isolate a problem in one particular channel.

After reading the Stereophile article and viewing the measurements, well... let's just leave it at that.

Interesting diagram, and that means I’m out of my experience level. I’ve never seen an amp "bridgeable" like this before, but the high output impedance of the transformer probably makes it less frightening than a SS amp with feedback coupled output stage

@erik_squires , @georgehifi When using tube amplifiers you can just parallel the outputs (and inputs) regardless of whether feedback is used or not. The Dynaco ST-70 instruction/assembly manual details how to do this.
Solid state amp are usually bridged (so the input to one channel is inverted from the other) as the output section of one amp is otherwise loaded by the other- leading to bad smoke.

For the former, ’monostrap’ is the correct term, for the latter, ’bridged’. The terms get conflated.

@psqt I take it then you have enough channels (four) to do stereo. Al makes a good suggestion as the inputs of the amps are also paralleled; if your preamp isn’t up to it you might hear some distortion, but I would expect that to be accompanied by reduced bass performance. I think something else is afoot- check all your tubes and make sure they are OK. If not the amp will lose delicacy and detail and may even distort in unpredictable ways. IOW I think you have a bad tube somewhere.

Thanks to everyone, and @atmasphere I am glad you chimed in (was hoping you’d be lurking here :) I am using a Backert Labs Rhumba Extreme preamp with 2 unbalanced outputs (L 1 and L2 going to the left Luxman, R1 and R2 going to the right Luxman - pictures in album linked below).

Also, all 8 KT88s have held bias steady around 485-ish since I first checked them last week.

I have had some time today to disconnect all connections, run in a single stereo configuration, then reconnect everything and give it some time to warm up, etc and make comparisons.

What I mentioned hearing before - I think it could have been my ears/imagination playing tricks on me, or simply something not properly connected or loosely connected. Everything actually sounds as it is supposed to in the parallel configuration.

What I am hearing now vs the stereo comparison is a more spacious sound field, more power in the bass in particular, and generally a stronger, beefier signal. I’m listening at about 90dB.

On the downside, it seems there is slightly more distortion in the sound running the 2 parallel amps. If you didn’t do a direct comparison it’s not likely you’d hear the difference. But there is slightly more "noise" around things like vocals, acoustic guitar plucks, etc. Again, it could be a difference in the age of the tubes, etc.

What I did was put together a small gallery of documentation for this process.
https://jsqt.smugmug.com/Luxman-MQ-88uSE-Diagrams-Measurements/n-cgz5rM/

It includes the actual block diagram of the MQ-88uSE (these amps), the specs, the instructions from Luxman on running the "C" version of this amp in parallel, as well as some additional measurements from HiFi News (UK) on this specific amp which aren’t exactly matching JA’s measurements in Stereophile.

Anyway I wanted to put all of this info together as a reference for this thread and if anyone else has ideas or questions about running 2 stereo tube amps in parallel.

Thanks again for all the info!

@jsqt, thanks for providing the additional info.

It appears that your preamp has very low output impedance, as the non-Extreme version is said to have a nominal output impedance of 75 ohms. So the somewhat low input impedance of two paralleled channels of your amp shouldn’t be any problem for it, even though the two RCA output connectors the preamp provides for each channel are almost certainly just jumpered together internally and are supplying the same signal. And in any event, as Ralph (Atmasphere) and I indicated if impedance issues were present at that interface you most likely wouldn’t be getting the good bass response you’ve described.

Also, as George and Ian (ieales) suggested trying a vertical biamp configuration could very well be worthwhile.

Enjoy! Regards,
-- Al

Thanks Al, I am curious about the vertical biamp configuration just for the sake of trying out all my options to see which sounds the best. This configuration would have benefits by running twice the output power split between the top and bottoms of the speakers?

The Volti Rivals are 3 way speakers with an external crossover (currently using jumper between the woofer and mid-horn). So it sounds like I’d use the Left channel of one amp to power the mid horn and tweeter of the left speaker and the Left channel of the 2nd amp to power the woofer of the left speaker.

The benefit here could be less distortion and more power to the individual speakers, but the trade-off between the parallel config would be re-introducing crosstalk between the 2 speakers and in theory reducing the stereo separation of the mix (vs parallel config)?