oops... yeah the HD means it's just an amp head.

you can use just about any cab you like, as speakers are just speakers, and the amp just wants to see some speakers that's all... BUT there's one very important thing... the speaker impedance...  basically it's to do with how hard the amp has to work to make the speaker produce sound...  if you get the impedance wrong, then the amp may have to work too hard, or not hard enough... so the amp could blow, or the volume could be cut down a lot.

anyway, here's a Line 6 FAQ document that explains what you need to do... you need a stereo cab with 8 or 16ohms per side, or a pair of cabs, where each cab is 8 or 16ohms mono.

http://line6.com/support/docs/DOC-3833

I hope this answers your question

Rowbi