There are a couple of things to consider. If you want the most even bass response, probably 2 subwoofers will be a better choice (though it will depend on room acoustics and where you place them, and with 2, it will be harder to set things up properly). But if you want the deepest bass response, one better subwoofer will be the better choice. Which matters more to you, depth of bass or evenness of response? If the former, get one good subwoofer. If the latter, then *probably* 2 subwoofers will be better, if one sets them up properly, though even so, it will depend on the particular acoustics of your room, so there is no guarantee that it will be better.
Also, will you have money/desire to upgrade in a year? If so, one better subwoofer now would be a better choice, so that the upgrade will just be buying another one like it later on, instead of getting rid of two subwoofers (which will involve losing money, as used subwoofers generally sell for less than what you paid for them) and replacing them both with more expensive ones. Given the way most people are who post at sites like this, I think the best advice for most of them would be the one better subwoofer instead of two lesser ones, to save money on their next subwoofer upgrade.
As an aside, if you don't object to the shape, you can get slightly better performance for the same price with the SVS PC-2000 instead of the PB-2000. (Look at the SVS website and compare.) It also takes up less floor space and is easier to move. The cylinder shape is very practical and very good; I currently own 3 SVS cylinder subwoofers (all old, discontinued models).
As for clarity rather than loud, keep it at the level your receiver sets in its automatic setup and you should be fine. Many people turn up the subwoofer from that level, which would likely bother your wife, judging from what you have stated. My wife was skeptical about better subwoofers when I wanted to upgrade some years ago, but she is happy about the old SVS CS-Ultra
subwoofers I got back then, as I wanted deeper, better bass, rather than overemphasized bass. She hates boomy bass, but setting it up right with deeper bass is something she likes. If you turn up the bass to make things sound unnatural, then I can certainly understand someone not liking it. Of course, some people really like boomy, unnatural bass, and as long as they do not subject others to it, that is okay by me. The one, better subwoofer would get you deeper bass, and that is what I would choose in your situation.