What is the Adjectival form of Tuba

xylophonic isn't much of a word in the first place. It just so happens we're familiar with adding the -ic suffix to other words (particularly those with Greek roots) such as homophobe, telephone, myopia.

We don't normally derive adjectival words for musical instruments at all, so as Snubian points out, if we're lexically forced to come up with one we usually just add the -like suffix.

It's my guess many people wouldn't think of xylophonic even if they needed the concept in a single word, so I think you'd find xylophone-like quite often anyway.

tuba-esque

seems to be fairly common. (Try google.)

It makes perfect sense, and the slightly comic feel of "-esque" nicely matches the somewhat comic feel of tubas in general.

(I can't stop thinking about Steve Martin's L.A. Story now!)

Don't be shy. If it doesn't already exist, coin it! Speaking as a mellophone player "Tubaphonic" sounds great to me (if you're referring to sound rather than appearance - To me, Tuba-esque brings to mind the configuration of the plumbing, which could also be what you want.)

All those great words we now take for granted were new, shiny, and hot off the press once upon a time.