Re: Trout to Bass
Eunan, you can certainly catch bass on most any weight fly rod. It depends on the waters you're fishing. I've used a 3 or 4 wt for Smallmouths in some very small streams, so even they are not out of the question. In these streams a 12" fish is a big one.
I've also done a good amount of bass fishing with a 6 wt. and have even landed a 30" Striped Bass with it, hooked while fishing for SM or LM bass in the Susquehanna. That size rod limits the size flies, & how far you can efficiently cast them as you increase the fly size. That Striper was hooked on a fly tied on a 2/0 hook, not something that casts well with a 6 wt, but it can be accomplished with a short cast. The majority of the bass I've caught on a 6 wt, have been under 3 lbs.
The 8 wt is a good all around rod for bass, but in some places & particularly where the average size bass may be small, it will be overkill. I have an 8 wt, and use it for bass often enough.
I also use my 9 & sometimes 10 wt for tidal river bass. More so because there's a lot of heavy cover, so dragging even a 2 lb bass out of the weeds needs a heavy rod, and because there's the possibility of hooking Striped bass as well. Plus, the heavier rod allows me to cast some really big flies.
Since you have both an 8 wt & a 5 wt, that would be a good place to start, and you should be able to choose one over the other for whatever water you decide to chase bass in & either will certainly work.
Remember, no one likes to be behind the big truck, but that's better than being under it!