I think the live center could be at fault. What method do you use for truing your blank ends before turning? Those are things that come to mind as potentially being a cause. Mismatch between centers will not cause out of round. Before I moved my employment years ago I spent the first half of my working career as a Tool and Die maker. On an engine lather if you needed a taper you shifted the tail stock off center to accomplish this. This was built in just for that purpose. So we turned a taper between centers but never out of round. That's why I believe the out of round is coming from elsewhere. I know I am not the only one that said you wouldn't get out of round from a center to center mismatch. Just something to think about and maybe check in to. As for the Jet Lathe they sound sweet and I am sure you would be pleased with it. Good luck and happy turnings.
It is possible that it is the live center. I am definitely no machinist. The reason I haven't thought it was the live center is because I have used both the cone center as well as the mandrel saver center and the results have been near identical. The out of round is probably a thousandths or two and I doubt many other than myself would notice. =) It is very possible that the play in the tailstock is not the culprit. When you say that you made a taper by offsetting the tailstock, what that taper the same all the way around the blank. For example, if you were trying to get the taper on a pen blank, would that taper be the same all the way around the blank, causing a lip that you could feel between the blank and the bushing? The lip that is created on my lathe can be felt only part of the way around the the blank. It is flush with the bushing about 60-75% of the way around and then not flush the remaining amount. It may be that the word 'taper' better fits the issue that I am having.
As far as the Jet or the Rikon, the specs seem almost identical. Two big factors that are different though are speed range within belt settings and quill travel. The Rikon has 2.5" quill travel which would be great for drill blanks. The jet has 2 3.8" quill travel so it is close. The Jet has a bit more range within belt settings that the Rikon seems to. The Jet midrange it 110-1800 while the Rikon is 550-1650. According to the Wood magazine, the Jet has vibration problems. I had not read anything about vibration problems with that model until reading the magazine article. If vibration was not an issue, I think I would probably lean towards the Jet but then price is also a factor and with it being around $100 difference with the current or soon to be sales going on, it is a tough call, at least for me. =)
Nate