RCA cable lenght

There is no hard and fast rule about maximum interconnect length, because it is highly dependent on both the output impedance of the preamp, and the capacitance of the interconnects. The lower the better in both cases.

If you choose interconnects with capacitance of say 20 or 30 or 40 picofarads per foot (pf/ft) or less, and if you have a preamp with an output impedance of say 100 or 200 ohms or less, you'll be fine. If both parameters are significantly higher than that, upper treble rolloff may result.

Also, the resistance of the path through the shield from one end to the other needs to be low, or 60Hz hum may result. Most quality cables should be adequate in that regard, but the only way to know for sure is to try it.

Regards,
-- Al

Assuming the interconnect capacitance and preamp output impedance satisfy the constraints I described above, and the speaker cables are of adequate gauge to support the run length (the Transparent Cables site doesn't seem to specify the gauge of the Ultras, though), then there is no unequivocal answer based on technical grounds. And the long speaker cable/short interconnect vs. short speaker cable/long interconnect debate is an age-old one, with divergent opinions (as Stan alluded to).

My instinct in this case, though, would be to go with short speaker cables. Your speakers have an impedance which is quite low at mid-bass frequencies, and which varies widely across the rest of the frequency range. The tube amps will most likely have higher output impedance than your Bryston, and you don't want to have the speaker cables adding to that higher impedance any more than necessary.

If the sum of amplifier output impedance and cable impedance (resistance and inductance) becomes significant relative to the speaker impedance, two things will happen. First, bass damping will suffer. Second, in a situation like this where the speaker impedance fluctuates widely as a function of frequency, then an uneven frequency response will result. That is because the voltage losses across the cable impedance and the amplifier's output impedance will vary as a function of the speaker's impedance vs. frequency characteristic.

Regards,
-- Al