That's an interesting point you bring up there. I've had a look at some models now, and most Envy24xx cards I could find are only keyed for 5V operation, the only exceptions being the Revo 5.1 and 7.1, the Audiophile 192 and the Philips Ultimate Edge 5.1 with universal keying. 5V-only cards are apparently very common, with universal ones being the exception rather than the rule outside of Creative. I do not consider this to be much of a problem, though, since even workstation/server-class boards tend to have at least one 5V-compatible 32-bit 33 MHz PCI slot, and ordinary ones are usually 32/33 (5V tolerant) throughout.
BTW, I'm wondering whether a service manual on the SE-150 PCI will be available; this is common in the hi-fi sector where Onkyo comes from but pretty much unheard of in the PC world. Whatever, a soundcard from a hi-fi maker (which shows immediately) certainly is an interesting novelty item and may attract some collector value pretty much regardless of how it performs.