Okay so I have a POD HD300 and I currently use it with my Vox VT15 and to be honest it sounds horrible. I am just about to start performing live with my new band and I was wondering whether to get a tube amp like the Bugera V22 and use the 4 cable method or go for a Tech 21 Power Engine.

Have you any suggestions ?
If you buy a tube amp it's best to have the option of going into the FX return, so try to get one with a loop. Also WRT going into the front of an amp try removing the cab sim from the amp model or even not using an amp model. The VT15 is a modeling amp so you can try running into the aux in if it has one.

Cab sims have an extreme EQ on your tone. So you should always try changing them or even bypassing when tweaking your tone.
There are some great tones on tap with the HD300. Like the previous comment, remove the cab sim. Actually, remove everything. Start from scratch.

Getting a real, great cabinet will set you in the right direction, because you essentially have an amp head with the HD300. I don't care if its digital, it works and there are many artists and albums with great tone to prove it. The power engine might be cool. Or grab a rack power amp and a normal cabinet.