Need an audio brainiac...Stumped...

Strange lack of synergy? It's got to be the output transformers reaking havock when paired with those crossovers. Impedance issues perhaps?

inductance or impedence...wow.

Well, it really could be anything, right? Two additional impeding mechanisms have to be taken into account when talking about inductance besides the normal resistance of DC circuits: the induction of voltages in conductors self-induced by the magnetic fields of currents (inductance), and the electrostatic storage of charge induced by voltages between conductors (capacitance). The impedance caused by these two effects is collectively referred to as reactance and forms the imaginary part of complex impedance whereas resistance forms the real part.

You are dealing with transformers in your amp output circuit, right? Maybe somehow they are getting saturated?

You might want to ask this question over at www.diyaudio.com

There's lots of guys over there with the knowledge to give you some real, valid guesses backed by experience.

This Wikipedia writeup seems relevant.

My suspicion is that there is nothing wrong with the speakers, and that some marginal instability in the amplifier is being brought out by the particular impedance characteristics of the Santana.

The following statements in the writeup particularly grab my attention, especially given that you have modified the original design to use a tube high voltage rectifier:

Low frequency oscillations like motorboating indicate that some device or circuit with a large time constant is involved, such as ... the filter capacitors and supply transformer winding.

One common cause is feedback through the plate power supply circuit.[2][4] The power supply provides DC current to each tube's plate circuit, so the power supply wiring (power busses) can be an inadvertent feedback path between stages. The increasing impedance of the filter capacitors at low frequencies can mean that low frequency swings in the current drawn by output stages can cause voltage swings in the power supply voltage which feed back to earlier stages,[2][4] making the system a subaudio oscillator. This is caused by inadequate power supply filtering or decoupling.

Regards,
-- Al