I used to mess about with the riff on the bass... 
That's very cool then. Most of my riffs from Hendrix, which were usually bass rather than guitar riffs, came from the first album. That's pretty much how I learned bass in the first place. Probably one of the main reasons why I also still feel such a connection to his music. This is where it all began for me.
Note:
No one seems to know any of this for sure. But it would be my guess that much of that bass on the "Are You Experienced" album was Jimi and Chas telling Noel what to do. He didn't even have his own bass until the album was almost finished. On the "Axis: Bold as Love" album, I think Noel came up with more of his own bass lines. He has even said that that was his favorite album. However, with a few exceptions, the bass on that second album is often easier than the bass on the first one. By "Electric Ladyland" when Hendrix didn't need Noel as much in the studio, that's at the same time he was actually getting a lot better on bass, IMO.
Like I mentioned, I'm looking at Billy Cox in a new light now that I've heard him on some of the songs I'd never heard him play on before that are on the "People, Hell, and Angels" album. I'm listening to him more closely on some other CDs I have. I'm about to listen to the Live at Berkeley concert again. Of course, when it comes to Hendrix's music, it's mostly the guitar that you'll be listening to whether you try to or not. In other cases in listening to music, I can sometimes almost ignore the guitar.
