I bought a Hiwatt Custom 100 and my bass player bought a Hiwatt 200 in the mid 70's after seeing these bands live: King Crimson, ELP, Pink Floyd, Jethro Tull, Led Zeppelin, and The Who. All of them using Hiwatts. All of them sounded great.
John Entwhistle, John Wetton, and Greg Lake all sounded fantastic using Hiwatts for bass. Really hit you in the chest.
I still have my Hiwatt 100 and would never get rid of it. It has a beautiful clean sound, kind of like a giant Vox AC30. I still use it for both guitar and bass.
If you want to use them for anything other than a clean (or slight edge of breakup) sound you have to use pedals with them, as even turned all the way up they do not have much distortion at all. You have to know how to use them.
David Gilmour has no problem getting great sounds out of Hiwatts.
Fremen has a good sounding Pink Floyd Axe FXIII presets pack.