This one gets asked alot, generally speaking mp3 quility comes from bitrate (or quality setting), but in addition lame has different internal encoding algorithms (as set by dBpoweramps Encoding setting in mp3 (Lame)).

These switches are set on lame command line with 'f' and 'h' and are documented as:

-h higher quality, but a little slower. Recommended.
-f fast mode (lower quality)

For R13 dBpoweramp matches lames mode with:

Normal = Lame default
Slow = Lame High Quality
Fast = Lame Fast Mode