It's not so strange. Tube amps have relatively low output current and low damping factors which make it more difficult to control woofers at low frequencies. The impedance of the speakers plus the cables affect the damping factor. If high capacitance cables are used, the impedance is higher at low frequencies, increasing the output impedance the speaker "sees" which makes things worse for low end performance.
tube amps/low capacitance cables?
The owner's manual for my 35w p/p tube amp recommends low capacitance speaker cables - but, it doesn't say why. On my two way monitors, I've been using TG Audio HSR silver cables which had been delivering authoritative bass in my former SS amp set-up. But, with the tube amp, the bass is now very weak. So, I'm assuming that these HSR cables are high capacitance, low inductance. I swapped in a pair of Speltz non-twisted anti-cables(low capacitance) and the bass returned. Since assuming can be a dangerous thing, can anyone elaborate about low capacitance cables working better (or not) with tube amps?