If the Riptidz don't work out, I would suggest Klipsch S4i.  You can get them for under $60 which may be more than you wanted to spend, but it is a great value for what you are getting.  Klipsch has oval ear tips which stay in the ear much better when you are moving and being active.  Plus I...

Very nice review.  Never tried the HD380's but the review makes me want to get a pair.

Do you have it narrowed down to those two choices or are you willing to look at something else?  If it is just between those two I might just go with Sony and save the cash.

You could try headphones.com.  Don't know if they will be any cheaper than amazon but you could try.  You might want to look at a used pair in good condition if you want a considerably lower price than what you've been finding.

Check the link below.  It is a comparison of the hd595 vs. the hd598.  Should be some useful stuff in there for you if you are deciding between the two.   http://www.head-fi.org/t/510701/sennheiser-hd-595-vs-598

I think it's between the XB500 and grado SR80i.  You would do well with either of them.

Audio Technica ATH-50's are decent headphones and I wouldn't have an issue suggesting them.  I would, however, recommend Grado SR80i over the ATH-50's.

I want to celebrate the HD600's too!  Excellent headphones. 

You should take a look at the Sennheiser HD595 if you haven't already.  They have already been mentioned by someone else but I think they would be a really good option for what you are willing to spend, how you will use them, and the genre of music you like.

Congrats on the new HD518's. They aren't the best but they should serve you well.

I agree. The D2000 is by far the best value for the rich bass you are looking for.

A few people have already suggested SR-80i's and I can't help but agree. To me, and a few others here, they seem to be a good match for what you are looking for.