MC Cartridge suggestions for Rega RB300?

I am currently using a Denon DL103 on a Rega RB300 / Michell turntable and I am looking for something better, preferable with more power in the low frequencies. Any questions in the 500-700 USD Range?

You might want to read about the counterweight upgrades available for the Rega RB300 arm, because it will improve all parts of the sound spectrum, including optimizing the bass.
I use Kerry Audio F2 Titanium Heavyweight Counterweight with positive effects that were quite obvious to all who heard either my Grado or Shure MM cartridges, both before and after. Unfortunately, the Grado is an old-timer no longer made, and the moderately priced Shure V15VxMR now out of production, with remaining stock moving up in price, though still below your price range.
I defer to others regarding your precise question about which MC design to provide best bass.
Please post which phono preamp you will be using since this helps determine perceived bass quality.
I have to say that I never noticed my 103R being lacking in bass but I was using a different arm and table so I'll take your word for it. Benz Glider comes to mind and being a decent tracker and good LF response. There is a high, medium and low output model. Probably not as detailed as the Denon but still a good performer for the money. Have you thought about a Shelter 501? Seems to be many happy owners of that cartridge out there. I haven't tried one yet.
I got the vpi jr with the 300b arm. I had the spacer and arm rewire done;---got a glider as well. 'Almost forgot when I went to the xlo 3.1 cable---big help there,as well. 'Cause phono cables matter.
I've been using a DL-103D for several years with an RB300 and now RB600 tomearm. Bass is lean with the stock Rega counterweight. It fills in nicely with the Heavyweight and presumably with the Kerry Audio titanium iteration. Also, my current favorite, the Michell Tecnowieght brings the bass into balance too. You have to spend more than 800 clams to do better than a Denon 103.

Thank you all for the suggestions, I will definitely look into the counterwight issue... I was under the impression that it was applicable only for the RB250... In any case I have already ordered the VTA adjuster ring from Michell and I will probably send the arm to a guy in Germany to change the cabling for about 200 USD.
My phono preamp is the Audio Analogue Aria MM/MC.
Ha! Thanks for posting 213Cobra! I do have a Technoweight on my modified RB250 so that's perhaps another reason why I haven't heard a lack of bass with the 103R.