Agree w/ Tpreaves, and you really haven't given any indication of what type of sound characteristics you're looking for other than bass extension. At the very least it would be helpful to know what it is you're looking to improve over the M60s (other than bass of course). The other thing is that there probably aren't that many speakers that go that low AND DO IT WELL in this price range. Or, put another way, you run the danger of getting quantity of bass but not necessarily quality.
OK, all that said, off the top of my head I'd recommend stretching a bit and looking at something like a used Silverline Sonata or Revel Performa F52 (either of which would be a huge upgrade to your M60s assuming they sound good to you).
The other way to go that would open a world of possibilities and probably give you even better bass performance is to look for a good pair of used monitors for about $1000 and a pair of good, used subs. I was thinking along the lines of Revel, Dynaudio, etc. but there are a whole host of choices for good monitors in that price range from a wide array of quality manufacturers. Point is the more choice you have the more likely you are to truly find "the best" speaker for you rather than potentially having to accept less optimal qualities (again, for your tastes) in other performance areas just to get the bass you're looking for. Then you can add excellent bass capabilities to the monitors of your choice by adding the subs (you can use products from companies like SVS or Velodyne to help with integrating the subs with the monitors and your room).
Anyway, I've gone on too long but just wanted to pass along those thoughts. Best of luck in your search.
OK, all that said, off the top of my head I'd recommend stretching a bit and looking at something like a used Silverline Sonata or Revel Performa F52 (either of which would be a huge upgrade to your M60s assuming they sound good to you).
The other way to go that would open a world of possibilities and probably give you even better bass performance is to look for a good pair of used monitors for about $1000 and a pair of good, used subs. I was thinking along the lines of Revel, Dynaudio, etc. but there are a whole host of choices for good monitors in that price range from a wide array of quality manufacturers. Point is the more choice you have the more likely you are to truly find "the best" speaker for you rather than potentially having to accept less optimal qualities (again, for your tastes) in other performance areas just to get the bass you're looking for. Then you can add excellent bass capabilities to the monitors of your choice by adding the subs (you can use products from companies like SVS or Velodyne to help with integrating the subs with the monitors and your room).
Anyway, I've gone on too long but just wanted to pass along those thoughts. Best of luck in your search.