dazz ok, an rc network is a resistor capacitor network. in my case the rc filters above 1.6 mghz to reduce rfi and emi. perfect for city use due to radio stations and high power lines. the network also applies a 10 ohm load at 100 khz. all speakers become more of an open circuit the higher they go in frequency (unless they use a similar network or the amp is terminated with a zobel) the amplifier sounds strained, resonant in the hf and far from the best it can because it is, unstable. using a termination or hf load will settle it down so it can sound its best. if your amp is terminated with a zobel on its output, do not use the rc. if it is un-terminated, then use it. if you use my rc with a terminated amp (depending on the freq of termination and amount of impedance applied), you may hear a resonant top end. two filters in one circuit with inductance (speaker wire) between them makes a tank or hf resonating circuit. no harm will come to the amp but the sound will suffer. this resonance will stick out in portions of the bandwidth and may un-weight the sound. that is why you heard more bass without the rc.
this device does not alter the impedance of the speaker within its audible bandwidth and it matters not, if you use tubes or ss.
best, b@merlin
