Tube Amp out of Bias-what does it sound like?

Just wondering, If you have a tube amp which is not biased properly, what effects on the sound quality would it have?

I have some amps which doing a bias adjustment is rather difficult, the amps work and are operational and may be close to correct bias, but wonder what the difference would be if all was 100% perfect.
Proper biasing can make a sonic difference. As little as a 5ma difference lower than optimum can change my sound from neutral/warm to neutral/cool. 5ma over optimum bias can shorten tube life quite a bit. Bais requirements change as the tubes age and, depending on use and tube types, should probably be checked at least every 6 months (I check mine every 2 months). Checking bias will also give you advance warning of an aging tube - you can change it before it dies or shorts out. The latter is well worth avoiding.
In a nutshell, if they are biased too low the sound will be groggy and not very involving, too high will sound better but you will put additional wear on your tubes.
I agree with what Newbee and Philojet have said.
Out of bias does affect the sound. That being said, it is unwise to say to low or to high. To much room for confusion. Much more appropriate to say to cold or to hot. To cold there is not enough current in the output stage to work correctly. Sound will be buzzy, not clean etc.. To hot can sound very good (unless way to hot) but lots of drawbacks. Tubes wear out much to quickly, the whole system is stressed to much which isnt good.
How long should you wait after turn to check/adjust bias?
The initial bias setting is made when the amp has been on for a few minutes, usually after a tube change etc, and with the bias set at its minimum setting. Once that setting has been made let the amp run for an hour or so to warm up sufficiently then go back an check/reset the bias to the factory recommended settings. Unless you have a bad tube(s) or a poor amp these last setting should remain stable for at least a few months. Good to watch bias, but no need to be anal about it. :-)