Hi, the question you are asking is largely about personal preference - the sound you want, the music you play, etc.
I'm going to recommend you look at 100W Roland Cube. But since you seem to be a detailed guy, let me add some detail.
I haven't used the Cube RX. I looked at it once because I was interested in the battery capability, but it's really small (something like 5W) so I didn't bother to try it. I use a 100W bass cube and really like it for small gigs or gigs where there is PA support and I just need a little extra onstage monitoring. It works well there. Similar amp modeling to the RX, which I like. The effects are there and switchable, though I don't use them because they are pretty basic. If they made a 500W Cube I would look at it for large stuff. Don't know if the 100W Cube fits your price range, but if it does it's worth auditioning.
I don't know your Ampeg option but I have used Rumbles. A friend buys them for school bands he runs as they are reliable, cheap and do a good enough job.
Hartke and aluminium cones....try it. I have heard some really nice Hartke stuff and based on that I bought a Hartke 210 box mail order. It's well enough made and does a good job the way I use it (paired with a single 15) but it doesn't have enough low end for me used alone. Likewise I tried the Hartke combo once and same reaction, so it wouldn't surprise me if the speakers were the culprit. I have heard their amps sound great.
You asked about tweeters.....depends on the sound you want. I tend to like tweeters if running double bass, but not so much if running electric, but I tend to go for a fairly old-school midrange sound on electric. The Cube doesn't have a tweeter.
Size - Rickenboogie said at least 300W and in the long run for a rock and roll gig I think he's right. Even 300W is small in some settings. On the other hand lots of us don't want to lug around an amp the size of a small fridge to get our sound, and the dangers of permanent damage to your hearing are real. Given your current needs I wouldn't think about all that just yet. I'd buy what meets your needs now and trade or get a second amp later. (Lots of us here have more than one rig). The other thing is, some guys want a lot of sound on stage and others don't. You can use a much smaller amp, or no amp at all and rely on foldback, if you are in the second category.
Parametric, sure. The fact that they are not so common tells you lots of players think you can get a good sound without them.
SansAmp is a preamp as well as a DI. It has quite a nice sound, which you can then send to the PA, and most models are programmable so you can have a couple of sounds set and switch between them. From where you are, it's worth looking at because it can make a cheap amp sound much better. I use one in front of my Peavey TNT which is rock solid but bland sounding, and it transforms the amp. I wouldn't bother with the SansAmp if you are buying a modelling amp like the Line6 or the Cube, but otherwise it's definitely worth considering.