Sorry, but your assumptions are not completely accurate. Modern components operate in what is known as "unloaded" conditions, and this is quite unlike the older days of tubes and transformers where impedance matching was vital for maximum power transfer (power transfer was important as it is intimately related to voltage transfer).

Simply stated, in an unloaded condition you want the input impedance of the receiving component to have at least an order of magnitude higher impedance than the sending device; in other words a 1:10 ratio or more. In the case of a component with a 1000 ohm output impedance, there will be no interaction (frequency response error etc) as long as the receiving device has an input impedance of at least 10k ohm. All currently available consumer components easily meet this requirement. I know for certain that the Outlaw amps meet this requirement by a large margin.

Interactions can occur with the capacitance of the cable, but with the 1000 ohm output impedance you refer to, it would require a cable capacitance of .008 uF to have appreciable drop in response at 20kHz. For reference, a typical coaxial cable has a capacitance of roughly 30 picofarads per foot - 30000 times less, and that's far too little to have any effect.

Modern preamps almost universally use operational amplifer ICs in their gain and buffer stages, and these have exceptionally low output impedance. The effective output impedance of these preamps, except for the most exotic, is determined by the resistance of buildout resistors prior to the output jack. These are included to provide short circuit protection and to provide electrical isolation, which helps avoid ground loops.

I'm afraid that that 1000 ohm output impedance of the 990 is inconsequential as far a having any ability to influence the sound you are hearing. As long as you are terminating the output of the 990 with at least 10,000 ohms, which is the case in almost all common consumer gear, and your interconnects are not hundreds of feet long (unlikely) there will be no audible "veiling" of the sound you are hearing.

Tube power amplifiers sound the way they do because of their distortion spectra, not their output impedance or lack thereof. Frequency response deviations can occur because of simple voltage divider interaction between the output impedance of the amplifier with the speaker impedance, but these are not what gives a tube amplifier their unique sound. Not all speakers offer a load which is complex enough to have interaction with the output impedance of a typical tube amplifier to a significant degree. In the case of systems which use active crossovers and consequently do not have passive crossovers within the speakers at all, the load is quite simple and response variations because of output impedance interaction are inconsquential. In any event, if somebody has a speaker which presents a complex, low impedance load, I would not recommend that a tube amplifier be used at all.

Tube amplifiers produce predominately low order distortion components which are dominated by the even harmonic products. These usually are greatly diminished by the 5th harmonic. A typical solid state amplifier, particularly a "balanced" amplifier (more accurately called a "differential" amplifier) will have an almost total lack of even order harmonic distortion components, with the total harmonic spectra having significant contributions well beyond the 10th harmonic. The even order harmonic components are important because they tend to mask the presence of all other harmonic distortion products. This is the reason that a tube amplifier can be producing, say, 1% total harmonic distortion, yet the ear cannot tell that it is producing distortion at all.