Tube Amp Warmth
Some A/V receivers do indeed have digital amplifiers, but many others have analog amps that use solid-state circuitry, and audiophiles argue endlessly about the sonic difference between digital, analog solid-state, and analog tube amps. In general, tube amps do have what is normally characterized as a "warm" sound due to the predominance of second-order harmonics.
Most modern AVRs have preoutsthat is, outputs from the preamp stage of the signal paththat can be connected to tube-based power amplifiers for those who want that warm sound. In this case, the signal does not pass through the AVR's power-amp stage, so the signal is not being "double amplified," which should never be done no matter what type of power amps are being used.
If you want use tube amps for their warm sound, it's probably better to get a preamp/processor rather than an AVR, since you'd be paying for amplifiers in the AVR that you won't be using. But keep in mind that tube amps are generally pretty expensive, and their output power is relatively low, so you need high-sensitivity speakers. And if you're powering a surround system, you'll need several tube amps, since there are few if any 5- or 7-channel models available.
If you have an A/V question, please send it to email@example.com.