M.
I didn't think you'd wired it incorrectly, but I gave the instructions anyway just int case - and it that vane you should probably check all is in the correct 'phase' also - that is all the reds are +, and all the blacks are -, at both your speaker and at your amp.
I gave the detailed description because you said the bass was low - really single wiring, or bi-amping, the level should be the same, so I think the problem is at the amp end - you may need to check those Advanced settings again.
Finally the mid-range of sound is arguably the most important to listening to most music - and the Bass speaker draws the most current and power, thus that is why I separate the mid and Bass so the mid is not adversely affected by large current draws due to the Bass. I wonder why Andrew suggested the other way? Yes separating Treble from the others may make it sweeter, but this is a very small part of our hearing range, especially in a 3-driver speaker where the tweeter may start at a higher frequency than a tweeter in a 2-driver set-up. Ultimately once you've got your amp working so the sound levels are correct again you should try both options and tell us what you hear.
You are right that the change may not be all that great - maybe for some speakers it won't even be noticable, but I know in my experience, bi-wiring my CDM1s was a good as a major amp upgrade!
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