Simple Biamp question

I know there have been dozens of biamp threads, but my question is so simple that I'm sure someone can answer it instantaneously.
I have a preamp with 2 pairs of outputs and 2 identical stereo amps than cannot be bridged into mono status. So I wish to biamp my speakers (Allison 4s with biwire posts)
1. Can I vertically biamp with the left leads of both outputs going to one amp and the left speaker and vice versa with the right leads? I have heard from some Audio people that I cant do this. Why? Something to do with crossovers and imaging and soundstage....
2. Is it better to horizontally biamp witjh both amps sending single wires to lower frequencies and both amps sending single wires to upper frequencies?

I would prefer the verical, as it's neater and easier, but are therte drawbacks? Is there a reason both left and right outputs from the preamp cannot go into one amp and use it as a mono?

Amps : TAD 60
Prreamp: TAD 150 Signature

Thanks very much for your help.
Have not tried it but either approach is valid. What I have heard over the years is that if you go "vertical" as in one stereo amp feeds the left speaker and the other the right channel you will get the benefits that are obtained by mono blocks which is the complete seperation of the channels at the amp stage of your system.

I defer to others who have spent time trying it both ways.
If my phrasing is misleading, let me clarify: (Vertical biamping):

Output A: L eft to Amp 1
Right to Amp 2

Output B: Left to Amp 1
Right to Amp 2

Amp 1 has 2 left input, Amp 2 has 2 right inputs

Is this doable/acceptab;e - likely to be a problem?
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.

-- Al
I vertically biamp using a TAD 150 sig to 2 SS amps. In my case though since I use one of the outputs for HT passthrough (this is how they are wired - one can serve as HT pt via resistive coupling), I use the single output and a Y-adapter at the amps (saves a long IC run).

I asked Paul Grzybek (RIP) about the impedance issue and he indicated I may lose some high end, but not likely. I haven't noticed any degradation at all and this Vert Biamp config is somewhat better than other amp setups (more expensive stereo) I have tried.
Why would my very respected audio repair shop tell me that the combination of 2 tad 60s and Allison 4s just won't work? They cite impedance and issues related tobthe Fact that the amps are stereo, not mono, amps. Any reason I am unaware of before I blow them both up?:)
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.

-- Al
Al, thank you for your valued input. Why would the absence of independent crossovers make them unsuitable for biamping?
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.

-- Al
Vertical biamping works wonderfully with my Monarchy SM70 Pro amps. Sounds much better this way than using them as mono-blocks, for some reason. I am very happy with the setup.
Al, I think that's at my repair guru implied - I misquoted him about stereo vs mono- so I am afraid of blowing up both amp and speaker by shorting the 2 channels. I have just asked in a new thread whether anybody has biamped Allison 4s. Thanks again for your input.
Because the Allisons are not made any more, it's difficult to find out what the crossovers look like, so Larry, my consultant repair guru, is just not sure how safe it is, but not based on any knowledge of the schematics, as he has not seen them. Hopefully someone will answer affirmatively , otherwise I may have to skip this experiment!
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.

-- Al