Running real recordings is better than testing. So yea, give it a try.
the standard addage is the closer the source the more impact the tool. Thus a mic will have more influence than the preamp, which has more than the converter, and all of which have less to do with the sound than the performance. My vote would be to put the money into the mic, tho I'm not sure how much more bang you are going to get for a couple of hundred extra dollars. I know that I'm pretty sure I could hear the difference when Michael Joly modded my 319. But I didn't really appreciate the difference between my old FirePod and new TCK 48 until I had used it for a while. A lot of it frankly, has to do w/ your ears getting more attuned to recording or mixing.
But I've learned that it is never bad to spend money on good equipement - you can go overboard, of course, but something solid like the ISA One is a great investment. I take it you haven't tried any new preamp w/ the ksm32 but are using your interface. I like to mix things up - transformerless w/ transformer, that kind of thing. The Warm preamp is another solid midprice unit, but like I said above it is more vintage - thicker if that makes sense. I've also got a pair of Warm's new preamp, the Tone Beast, which has all kinds of variables - different transformers in the output, opamps in the mic pre, etc. etc. It is lots of fun since you can mix and match w/in the preamp. I was recording bass w/ it last week (DI'ed) and I got a completely clean sound that worked on one sound while on the next song I put a growl into the bass using the gain/output knobs. I mean, who knows what you'll like. Again, your best bet is to try some stuff out yourself (you are recording your own voice, right).
As for the summit audio preamp - that is the one w/ the variable tube input, right? About $700? I've heard good things - I'm sure it is a capable unit. I must say I've never used the summit. Some of their other stuff, yes, tho I'm not really all that familiar w/ the line. But for that type of money ($600)and adding some variable sound the Tone Beast would be my pick - simply because I've used it - a lot. I got a second one for sending stereo signals as well as mastering. But I don't know if that kind of saturation is what you are looking for - the distortion you can get is better for instruments.
If you can't try before buying you are probably best off getting a good mic preamp and a good mic. I've used the ISA one and like it. I'm sure the Summit would be good, too. But it might take a while to fine the perfect mic for you at a reasonable price - you would probably sound great w/ a vintage Neuman. But in the meanwhile you won't be going backwards while you figure out what you want in a mic.
@