Brass Trio vs. Quartet, opinions please

[Mike Finn]

I'm currently playing the bass bone part (on tuba) in a trombone trio. We've talked about adding a 4th voice since the beginning, but wanted to make a go as a trio first. But now it just seems like there are too many empty spaces, like something's missing. One option would be for one of the bones to switch to Flugel, which would increase the range and change the sound of the group, but I'm afraid we'd still be missing that fouth voice.
I guess I'd really like to hear from folks who either are making the trio thing work, and what arrangements you are using, or from folks who tried it and decided that the quartet was better. Is it a whole lot better?
Thanks in advance for all opinions and shared experiences!
MF