Sounds like they're listening to the tubas bass drum, trying to time off of them. As you get faster, if they play off the tubas, they'll be late. They need to count their part, tap their foot (get rid of that by performance time), and play in time. Practice w/met, then w/o met, then w/tubas.

Another met trick - works great with a harmony director, dr beat, or another met with sliders for various beats. Turn on the met with 8th notes. Start foot taps on the downbeat. Slowly pull down the slider for the downbeat, so you're only hearing the upbeat click. Work on keeping the foot taps in time. Do the opposite with playing on the upbeats. Start the met with 8th note clicks, get the upbeats in time, slowly pull down the slider on the upbeat click until they only have the downbeats to listen to. Slowly pull that slider down so they have to do it internally. Then add tubas back in.