I have a secondary 6 string bass that is fantastic, all except for this issue. When playing the G and C strings from the 9th fret and up, there are resonating tones that can only be ceased by muting the B, E, and A strings. Any known solution to this?

Yep. Mute the B, E, A and D strings.

Seriously look into the floating thumb technique. This is one of the two problems it solves. The other being a painful wrist some years ahead. The more strings you have, the more sympathetic vibrations you get. If anything, it's a sign of a bass that's not mechanically dead.

Thanks for that tip. I'm reading a thread about it now. Hopefully, I'll be able to adjust.
Thanks again.

are you using DR strings (especially the uncoated kind)?

swapping from DR's to D'Addario's eliminated this same thing when I was experiencing it

all the best,

R

I have Dean Markley SR2000s currently on it

More comments?

I don't think there's anything you can do to the bass to stop this. I'd take it as a good thing, like Rune said it shows that your bass is resonating well. It happens on guitars too.

Think you'll just have to work on that "floating thumb" technique. (I'm not a bassist)

Sounds good. Thank you.