I have eyesight issues that dictate that I use a top-action tuba.
I live in Manistee, so a drive to Lansing for major service is no big deal.
My Holton needs its valves replated. I am contemplating adding a fourth valve to it--but the privileged tones are so strong and accessible as it is now, I'm not 100% sure it would be worth it.
I have seen one done as a 3+1 without a compensating system from the Holton factory. A friend of mine owns it and claims it was owned by Jacobs. Not sure if that is true but it does have 4 valves.