Simple Biamp question

That will most likely work fine, and should give superior results to horizontal biamping, for the reason Jjrenman cited and perhaps also because it would make it possible to use shorter speaker cables.

The reasons I say "most likely," though, (and these reasons are equally applicable to the horizontal and vertical biamp configurations) are as follows:

1)Chances are that the two sets of output terminals on the preamp are simply wired together inside the rear panel of the preamp, rather than being driven by individual buffer stages. If so, the output stage of the preamp will see a load impedance equal to 1/2 of the input impedance of the power amp. I found a spec indicating that the output impedance of the preamp is "less than 600 ohms," but that is probably at mid-range frequencies and could be much higher at deep bass frequencies. And I couldn't find a spec on the input impedance of the power amp. Given that the power amp is tube based, I'd feel fairly confident that 1/2 of its input impedance will be sufficiently high in relation to the output impedance of the preamp to not result in sonic issues, but that can't be said for sure without having more complete specs.

2)Assuming that the two sets of output terminals on the preamp are not individually buffered, the output stage will be loaded by the sum of the capacitances of two sets of interconnects. That most likely won't cause any problems, but if the interconnect lengths are particularly long (e.g., in the vicinity of say 12 feet or more, each), choose cables having low capacitance per unit length (e.g., 25 pf/foot or less). A slight rolloff of the upper treble might result if the cables are long and have high capacitance per unit length.

Regards,
-- Al

I have no idea why the fact that the amps are stereo would be an issue. The fact that they said that lessens their credibility, IMO.

One conceivable issue, though, is that there are some speakers that provide two sets of binding posts for biwiring purposes, but do not have low frequency and mid/hi frequency crossover networks that are independent of each other. In those cases, although the speakers can be biwired they may not be suitable for biamping. Don't know if that applies to the Allison Fours.

Regarding impedance, it appears that the speakers have a nominal impedance of 8 ohms, and the amplifier provides 8 ohm (as well as 4 ohm) output taps. So I would expect impedance to also be a non-issue. The one conceivable concern that occurs to me, though, is that the mid/hi amplifier channel would be running essentially unloaded (i.e., into a high impedance) at deep bass frequencies, and likewise for the low frequency amplifier channel at high frequencies. It is not healthy to run a tube amplifier having an output transformer unloaded, because it creates a risk of amplifier damage from something called "inductive kickback." But I see no reason to suspect that that risk in your situation would be any greater than in any other situation where a tube amplifier is used as part of a passive biamp setup, which is commonly done.

Regards,
-- Al

Why would the absence of independent crossovers make them unsuitable for biamping?

It is conceivable that if the speaker is not designed to be biamped its design could be such that it would short the two amplifier channels together, either directly or through some low impedance circuit path. That could be unhealthy for both the amp and the speaker.

In that situation the risk of damage would be reduced somewhat by the fact that both amplifier channels will be putting out signals that, to a close approximation, are identical. But if you don't have a manual for the speakers, or if the manual does not address biamping, it would be prudent to do some Googling to see if others have biamped that speaker successfully.

Regards,
-- Al

Take a look at the rear panel of your speakers, and see if there is a statement indicating "remove both links for biamplification."

I found this writeup, which shows such a statement on the rear of what is probably the Allison One, although the Four is also mentioned in the text. See the third photo from the top. If that statement appears on your Fours, you're good to go!

Strangely, though, I also found several rear panel photographs of Fours, which only showed a single pair of binding posts (as well as a three-position switch to control "acoustic power balance"). Perhaps there were multiple versions of the Fours.

Regards,
-- Al