Re: Looking for a new fly rod...
For me when I think of bass fishing it is either with a 6 or 8wt. Not so much for the size of fish you intend to catch, but the size of flys. It is tough to throw a Clouser minnow or deer hair popper on a 4wt.
That being said, I think a 6 wt is still a good all around fly rod for both warm water and trout fishing. For trout I tend to use a 4wt but still use my 6 when throwing bigger streamers or heavy nymphs or anytime the wind is blowing that makes casting a 4wt impracticle.
As been already mentioned, you can still cast small flys on a 6wt. You may have to lengthen your leader in order to step down to smaller tippet, but it still can be done. In the winter I will often take the 6 for reasons I just mentioned and if a midge hatch comes off do not feel that I can't present those tiny flys with the delicacy required.
The only thing that may be a challenge is finding an 8' 6wt. At least in a more modern rod. An alternative would be to look at a 8' 5wt. You can still use it for bass but again will be limited to how big a fly you can throw.