Tube preamps typically have a far higher maximum output level than the mass majority of SS designs. As such, the only thing that i can think of is that the tonal balance has shifted due to different power transfer / loading conditions and you seem to like those changes.
Other than that, a preamplifier is nothing more than an amplifier designed to work with a lower input signal and a lower output signal. Think of a preamp as an "intermediary gain stage". It can take a weak signal and boost it enough to drive the next gain stage or it can regulate a signal that may otherwise overdrive the next gain stage through attenuation. Pre-amplifiers got their name as they are nothing more than an amplifier stage prior to the main amplification stages, hence "pre" ( before or prior to ) amplifier. As such, most do not consider "passive units" to be pre-amps as they have no amplification stages. However, one "could" say that they go "before the amp", so in effect, it is still a "pre" amp.
Other than that, there are preamps with enough voltage swing and a low enough output impedance that they can drive speakers. They will typically sound like tin cans vibrating when doing so, but you might be surprised what comes out of the speakers if you were to try doing this. This is NOT a suggestion to try driving the speakers via a preamp even though i have. For the record, i did this as a "yout" ( pronounced "youth" ) when i didn't know better. Sean
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