Chris McKinley said:
A graphic EQ used in the effects loop, not in front of the amp, will do it if you lower the 200Hz slider to taste. "Tightening" an amp's low end means getting rid of the muddy sounding frequencies, which generally center right around 200Hz.
You can also use one of the many overdrive pedals available. While there are many models out there, there are two general categories they fall into. Some are full-range, in that they provide overdrive in a somewhat flat EQ curve, boosting bass, mids, and highs somewhat evenly. Others are variations of the TS-808 Tube Screamer overdrive, which tends to emphasize the upper mid frequencies more than the others. Because they reduce bass overall and thicken the mids, the Tube Screamer types will generally tighten the bass of your amp a little more than the full range types, but depending on taste, both can do it sufficiently. It really comes down to whether you want the kick in the mids (usually desired for soloing) or the full range drive which tends to preserve your original tone a bit more.
Unlike the graphic EQ and certain other types of effects pedals, overdrives are always used in front of the amp, and never in the effects loop. They are specifically designed to be used with a before-the-amp placement.
While an EQ in the loop would do the trick, he mentioned he's a "plug and play" kind of guy. Sticking on the FX Loop, fooling with the balance between Master and Output, finding just the right slider to move the right amount, etc...would seem a little too much work for a plug and play guy in my opinion.
@ Botch, I'd go with the OD pedal, and something in the Maxon OD808 or Tube Screamer vein as mentioned above by Chris. It's quick and easy, plug and play and fool proof. Put the Gain at "0" (off), the Output anywhere between 12 Noon and max, and the Tone to taste, though I think you'll find the sweet spot in the 9-10 o'clock range. That's the way probably 95% or more of guys are tightening Rectos when you hear them talk about using a pedal to do so.