The single driver Sure buds that came with my wireless monitor system worked fine for playing fiddle onstage, but don't even come close to meeting my needs on bass.
You're going to get an automatic improvement in bass response going from universal fit (even with comply foam tips) to custom molds. That alone might be enough for you, but I suspect not.
Are quad drive IEMs sufficient for most bass monitoring needs? Would there be a huge difference in the bass response/clarity/frequency range between 4,6, or 8 drivers? It would definitely make my wife a lot happier if I could say I saved a little and went with a less expensive set, but I'd rather pay a little more now and get the right set, than be wishing I had done so later...
There is a difference between 4, 6, and 8 driver IEM's, but some of those differences can be overcome through proper EQ treatment of the signal arriving to the buds. My experience with 1964 quads comes from using various "high quality" single driver universal buds (shure, etymotic, monster, klipsch, bose) and a set of Westone UM3X triple driver universal buds. The difference between 1 driver and 3 drivers was huge, and the only real difference between the UM3X buds and my quads was in the low frequency part of the signal. HUGE improvement with the addition of the sub armature! It was almost like I was feeling the bass notes. For me, anything beyond 4 is just gimmick for live use, but YMMV.
Second, I'm pretty concerned about the isolation bubble that comes from using IEMs.
You'll likely never overcome the bubble, but you can minimize it with a condenser mic routed into the talk-back line of your board or a dedicated channel or two that only feed the aux mixes. If your band is planning to go wireless, Audio Technica's M2 and M3 line of wireless monitor systems have the capability to add a shoulder-mount lapel mic that is volume independent of the aux mix so you can dial in as much room noise as you want.
Third, would bass response be more accurate from a larger single woofer(Westone ES50) or from 2 or 4 smaller woofers(v6 or v8)?
More drivers devoted to specific frequency are going to be better. Consider a high-fi audio system with multiple drivers and crossovers to handle distribution of the work...they always sound better than what's coming through a single speaker. This is why the quads are, IMO, better for bassits than v1 through v3.
Lastly, just about every review I have read has been very complimentary of 1964ears as a company. That seems to be the agreement here as well. Westone, has been around for 40 years and has a lot of experience, do you guys feel that 1964ears is on par in terms of service and quality? I don't mind paying more for service/quality/longevity, but I hate just paying for a name.
I've never dealt with customer service for either Westone or 1964, because I never had any problems with either product. Others may be able to comment, but Westone is a reputable company who stands by their products. To what length they stand I can't say, but I wouldn't hesitate to buy from them if I was set on their product and customer service was my only hold-back.
I've never been more happy with what goes into my ears than I am with my what my quads provide me, both from a live performance and personal listening experience. You probably won't be disappointed with the V6 or V8's either, but I wouldn't spend the extra cash unless you just want to. To be fair though, it's not like you can quickly (or cheaply) swap out models to compare, so go with your gut.