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

Wait, when you bridge an amp you run:

Amp L  (+)  ---> Speaker (+)

Amp R (+) ---> Speaker (-)

That doesn't sound like what you are describing. 

First, to be precise it sounds like what you are doing is paralleling the two channels of each amp, which is not the same as bridging. (Although the term "bridging" is sometimes misused to refer to "paralleling"). Bridging involves operating the two channels with opposite polarity signals, and connecting the speaker between the two positive output terminals. You are not doing that since you "used a jumper between each of the "positive" taps," and I presume you are supplying both channels of the amp with the same signal. Also, I suspect that this amp is not bridgeable.

An amp which can be bridged will usually provide a greater increase in power capability into 8 ohms or thereabouts than paralleling the two channels, compared to the amp’s power capabilities in stereo mode, but paralleling will usually be better able to handle low impedance loads.

In any event, it appears that the amp has a single output tap for each channel that is optimized for a 6 ohm load. By paralleling the two channels the tap would be optimized for a 3 ohm load. Looking at the impedance curve for your speakers that is shown in John Atkinson’s measurements it appears that the impedance magnitude varies between a low of about 3.5 ohms around 100 Hz and very high impedances in much of the mid-range and lower treble. The impedance being more than 20 ohms between about 2 kHz and 4 kHz.

Given all of that, I’m not sure what might be causing the symptoms you have described. With the amp presumably now optimized for a 3 ohm load perhaps it isn’t happy driving such high impedances in the mid-range and lower treble. Or perhaps the tubes or something else in the amp isn’t well matched between the two channels, and the two channels are trying to put out slightly different signals and are therefore fighting each other to some degree as a result of having their outputs connected together.

Also, just as an FYI, JA measured the amp’s input impedances as 54K and 34K for the direct and variable inputs, respectively. So by applying the same signal to both channels you are presenting the preamp with a load of 27K or 17K, depending on which input you are using. Some tube-based preamps will experience a bit of deep bass rolloff driving such low impedances, but it sounds like that is not occurring in this case.

Those are all the thoughts that occur to me at this point. Perhaps the background I’ve provided will result in additional possibilities occurring to someone else.

Regards,
-- Al

If you are attempting passive bi-amping:


Amp L ( + ) ------ Speaker HF (+)
Amp L (-) .-------> Speaker HF (-) 

Amp R ( + ) ------ Speaker Low F (+)
Amp R (-) .-------> Speaker Low F (-)


In neither case should the (+) amp terminals touch each other, even from matched amps. The slightest (0.001V) variation in output can induce significant current as the sides fight for the right value. 


Best,


E

Sorry, if you are using different output amps, your set up would look more like this:

Amp 1 - L  ( + ) ------> L Speaker HF (+)
Amp 1 - L (-) .------->  L Speaker HF (-)

Amp 1 - R ( + ) ------>  R Speaker HF (+)
Amp 1 - R (-) .------->  R Speaker HF (-)