It's more Hughes' playing than what bass he plays. He sounds similar whether he plays a Mustang, Ric, P, Jazz, Manne, Vigier or ESP. He hits those strings hard with his pick and does a lot of dampening and controlled vibratos with his left hand. Plus his ubiquitous bending and intentionally sloppy sliding. Funnily enough, he probably sounded the least distinctive with the Ric, which is a very distinctive sounding bass per se, but Hughes and Ric never gelled. He dumped it quickly on the first Burn tour after he had started out with it in the Burn recordings, most likely to lend some continuity in the bass department as Roger Glover had played a Ric in the last years of DP Mk 2. The Ric on the Burn album comes nowhere near as well out as on the albums Glover recorded with the Ric while in Purple (Machine Head, Made in Japan and Who do we think we are). By the time of Stormbringer, where you hear the bass well, Hughes was playing the California Jam P. Same on Come Taste the Band.
The amps might have something to do with it. Glover played Marshalls and the distortion they gave him (which he hated, he always wanted a clear or as he put it "American" bass sound) enhanced the Ric's midrange making it quite prominent. Hughes played Hiwatts which worked great with the P, but not so much with the Ric. His bass sound on Burn is if anything to deep with too little mids which is why you have a hard time hearing him whereas Roger Glover was always easy to pick out. And that is even though Hughes is the much edgier "I want to be heard too!" player whereas Glover is more melodious "flowing with the music", but not looking for attention. Notewise, Glover is actually busier than Hughes, Hughes is funkier and more ahead of the beat, but actually play less notes if rhytmically more prominent.
