Tendonitis, Carpal Tunnel, etc.. It's all down hill when you hit 40. Don't mean to scare you, but if your day job is your playing job, you have to take care of your health.
I know too many multi-course bass-players that have chronic tendenitis from the gymnastics required to play these wide boards, and usually applying a technique that is inconsistent with the dimensions of the instrument.
After fiddling with an Aria-5 I realized that these instruments require a refined technique as well as a double jointed or highly flexible thumb on the fretting hand. As noted before - adjusting your strap to accomodate the proper relaxed hand positions will aid in relief.
In classical guitar we are taught to position the instrument so the fretting hand does not take on a dual role as a support. The hand is relaxed, the thumb doesn't need to be gripping or pressing into the neck. The thum has to free because there are times where the technique requires the thumb visit the fret board.
I'd suggest stretching and slow relaxed warm-up's before playing any bass, and espescially before a gig.