[Sasha]

Would there be any negative effects if I use cables of different length for tri-wiring and by-amping?
Due to the position of amps, I may have to drive LF from one amp using single run of 10ft, and MF/HF using another amp with runs of 8ft.
Why not replace 8ft with 10ft you may ask, because of the cost of selling 8ft and buying 10ft, which I would like to avoid if possible.
Speakers are IB2 and amps are 4BSST and Linar class A.

[brucek]

Would there be any negative effects if I use cables of different length for tri-wiring and by-amping?

Electricity travels near the speed of light in a wire (depending on its velocity factor).
This speed can be approximated to about a nanosecond a foot. The accepted explaination would be that if you had one speaker wire 50 feet longer than the other, then that signal would arrive about 50 nanoseconds later than the other. Well, this is about 1000 times less than human hearing can even begin to detect, so don't worry about the lengths of your cables and phase differences.

brucek