best turntable for 500$ or less

This is actually a harder recommendation than it seems, not because analog has to nessesarily be difficult, but in your wisdom of giving us as much info as you have, it leads me to think your criteria are more stringent than you elude to.
Clue- your system is all Mac-while mac products are often accused of looking better than they sound (they are all some of the best looking stuff there is), one thing about mac is TASTE, not just in appearance, but in the sound mac reproduces. Mac's are expensive products, and where they have shortcomings sonically, these concessions to sound quality are the result of assuring that the sound reproduced is for lack of a better word, tasteful, which mac products do not seem to comprimse. Mac owners seem to demand this, this 'good taste' in not just appearance but in sound quality as well. The complexity and grade of your system lead me to believe that what you are looking for is a turntable that has these qualities, in sound as well as appearance.
If you choose to go the inexpensive route, an AR would be a fine way to go, and then there is the arm to consider. AR's floating around out there will have a variety of arms. If you are in this low price point it would be easier to consider what is available to you, what you can find, then to try to target specifics. A good cartridge for you might be a sure v15mr because it will allow you a good match with a lot of older, lighter mass arms that you may end up with, giving you more options, and it mates very well with mac phono stages. (in other words, this cartridge as an option will make more arms an option for you). Grado's are a great option as well, especially the cheap ones, and you can try a really cheap one to see if it hums in your system.
It is hard to comment on a denon because there are a lot of different ones, and then of corse if you are buying off the web you are more blind in what it is, but there are some good ones that can be had for 200 to 300 dollers that are worth that, but beyond that the AR is problably much better in performance, looks, and of corse you can more easily get another arm for it should you find the one you get unsuitable.
On to rega- the cheaper regas will work well but in your case of staying below 500$ for everything will problably not suite your sonic taste when you consider the comprimises you will have to make, unless you get lucky and find one that is set up with a cartridge that is exceptional and you get a really goon deal. While regas are really good for the money, the most you will problably get out of it is a good arm that you could uprade with. But, some of the better rega tables are a different story. If you are willing to stretch your budget, and put a little more effort in cartridge selection, the overall character of the sound of the regas will change from being a detraction on the cheaper ones to being an advantage in the more expensive ones, and will mate very well with your mac system, and the higher end rega's are also in a different ballpark visually as well. The effort and thought required to choose a good cartridge for a better rega is about the same or less than going the cheap route, just more is at stake. At least look up some pictures. Considering the care taken in the rest of your system, I suggest this considering spending more as perhaps having a turntable that better suites your taste.
Either way, if you let us know what you think about these recommendations (from everyone) after checking out what is available to you, I am sure you can get some more specific recommendations to end up with something that actually suites you quite well.

I have a couple suggestions regarding your dual/ortifon use. First, I don't know how old the cart is so keep an ear out for mistracting, which will sound like a buzzing or harshness during peaks or certain passages in the music. Also, if you are into experimenting a little, try a peice or electrical tape on the tonearm. stretch in lenthwise along the arm and wrap it around tight. The reason for this is the duals from that time were made with very light, low mass arms that were meant to be used with low mass carts, with the idea that the resonent frequency would be above the music, as opposed to how they are made today, which is the opposite philosophy. The result is the arms on duals ring like crazy. The tape will serve many functions- first, it may put the arm and cartidge at a better match, it will dampen the arm quite a bit (which may be a trade off evan if it makes the arm/cart match worse), it may help with tracking having that little bit of extra mass, and above all, having to readjust the setup with the tape and without and listening for improvements will gain valuable experience, as well as listening for various changes in sound qualities.
Also, if you can solder or are willing to do it, you can add a pair of radio shack rca's and wire the tonearm wire to them and experiment with different cables for phono. If you are succesful at this, rewire the tonarm leads at the headshell directly to the cartridge clips. (if you do this, be sure to locate the rca's in such a position to have enough slack as you will lose some at the headshell).
One more thing- keep an eye on the speed control mechanism, and experiment with different lubricants if nessesary.
With these, it may actually be possible to have the tt perform on the level of your cdp. Evan surpass it in some areas.

sometimes oil can cause more noise than grease. I thing I did use 3/1 oil before in a dual and it worked well, but if it doesn't, than I would try some type of grease- just listen for noise.