#1
A few months ago I joined a band, I am a bassist by nature and did not have a guitar amp, and one of the other members gave me an old line 6 spider. Since then I have discovered that it's the worst amp in the world and horribly inadequate for gigging. I've been saving some money for a little to get something much better - but the spider sounds terrible and the controls have started getting wonky, so I would rather take action sooner if possible.

Someone I know, their band broke up and he's not really interested in music, offered to sell me his ad100vht and an random unbranded cab for really cheap. Is this a decent head or is this thing garbage?
#2
Depends on the sound you want and the price.
The Vox AD series are modeling amps, but they are one of the better modeling amps (Worlds better than the Line 6 Spiders).
#3
We have the 20, 40 and 60 at school and the cleans are alright on the modelling amp, the effects are pretty tacky but the distortion channels are pretty bad

My favourite setting is the super reverb channel, but it has 22(i think) other amps on it. I wouldnt recommend it to you as a bass amp, its just not beefy enough
#4
If its the same one I have, it'll work if you're desperate but I wouldn't recommend it as a gigging amp or use it as a bass amp. And how much is your friend asking? I paid $100 for mine to give you an idea. Also, they're extremely muffled sounding when cranked.
Fender Mustang/Derfenstein DST> Boss Power Wah> Pedal Monsters Klone> Bogner Uberschall> Walrus Audio Janus> Randall RM20> Line 6 M9> Randall RM20