Is it possible to create a 4-channel tube amp specifically for car audio?
as sir tony said, yes. a hardcore DIYer can even create multiple inverters to supply the individual voltages needed by the circuit.Is it limited to only two channels?
it is limited only by space, and imagination. Do you think a DIY Tube Amp can withstand the rigors of Car vibrations?
yes, but it should employ serious vibration dampingHow big can it get? Or vice versa, how small can it get?
BIG
And most importantly, what's the estimated pricepoint for such an amp?
EXPENSIVE! the cheapest would be around 10k for a sakua & inverter. add the cost of vibration-proofing the unit and some spare tubes.
one possible problem is micrphopics, perhaps you can use that set-up while your car is parked and not while on the road...
Plus the sonic benefits of having a tube amp in a car can be easily masked by road noise. i won't consider having a tube amp in my vehicle unless i have a very quiet luxury car, or one that will fit two setups (valve & ss), both of which are very unlikely.