Sorry, I missed your budget when I was scanning!
With your budget, I would recommend three options:
The JBL 530's while they are on sale at $300/pr:
JBL Studio 530 versatile bookshelf loudspeakers deliver accurate powerful rich sound to your living room and can be used as left, right or surround channels.
www.jbl.com
These will put out a ton more powerful sound and have good energy for a workout room (while still being accurate for their price).
I would consider (based on my own listening, which does not include the Outlaw speakers that ShadyJ recommended) these $440 Canton speakers with the ceramic tweeters the highest sound quality option in your price range, but they are only available in white. However, I think if they are totally for workout, the 530 might better serve you!
www.accessories4less.com
If white doesn't work for you, You can stretch your budget to $520 to get a 1" larger woofer and in black:
www.accessories4less.com
Last if you are pining for towers, these are very close to the quality of the 530's and an incredible buy at $270:
Find many great new & used options and get the best deals for Pair of 6.5" Bi-Amp Home Theater Floor Standing Tower Speakers DCM by MTX Audio at the best online prices at eBay! Free shipping for many products!
www.ebay.com
The Daytons you have are not bad for what you paid, but all three of these speakers are discounted by better than 50% from their MSRP and all three of them were good/competitive speakers at MSRP.
I also have the R162 that ShadyJ recommends, but I would have to give the 530's the nod for a workout room. Actually, I think the 530's are the best fit based on what I know, but I don't know what else you might have in mind for them. The 530's are pretty efficient and I think they will work well with your Dayton amp!
Like I say, it is a very good time to be buying speakers! I think any of the above will keep a smile on your face!
Get the speakers first then gradually increase the volume on your Dayton while listening for distortion. The Dayton amp is pretty decent, but I don't know how loud you like it for workouts!
I do agree with LovintheHD on the bass management, but get the speakers first, and we'll go from there (obviously, a amp/avr with bass management will likely get you past your $500 budget - unless you want to spend less on speakers). The least expensive option with competent bass management I would recommend is this $200 unit (but a TV is awfully handy for the setup menus, even if you temporarily use it for setup):
www.accessories4less.com
Note that this gets you bass management, but not much more in the way of power. Significantly more power will pretty much require your total $500 budget for the AVR!
Maybe
@shadyJ knows about the efficiency of your Dayton speakers, I think he may have tested them?