Personally, I would be very upset if my gauges crapped out after only 6 years, bearing in mind how long automobile instrumentation lasts.

What do you mean by gone out? It seems most unlikely that both the speedometer & tachometer both belly up at the same time. Have you done any kind of troubleshooting? I would be looking for something common between them, like a ground, a fuse/circuit breaker. These gauges (just like everything marine) are expensive and not always easy to swap out unless you have the same models. The senders have to be compatible with each model. More information would be useful. I would contact the gauge manufacturer for replacements..since they are only 6 years old, I would guess that they will still carry spare parts, or replacements. Good luck.
Kev.