Suggestions for bookshelf speakers that have great dynamics, budget of $500 used.

   I listen to both Monitor Audio bronze and silver floor standers, and I don't find them to be too bright to interfere with enjoying music - perhaps it's system matching but I enjoy MA speakers - granted they are only mid-fi...I bought them not auditioning them before hand because of favorable reviews on an European audio magazine - they gave 5 out of 5 stars at the price point. I admit the silver line tweeter is a bit bright, but I love it for their piercing sound of soprano that goes to my heart...

What are ultimate goals for this speaker purchase ?   Will this be a garage-man cave rock n roll system ?   Do you expect to play challenging, detailed, well recorded music ?     What are your musical tastes ?

Various Klipsch models are probably a default choice.  Hersey would probably be the sweet spot, as a used pair can be found for less than $500 and are easily modified to sound better (if desired).  They are efficient, play to loud db levels, and are very good rock speakers....but not so good with other kinds of music.

If you want a plain head banger pair of speaker, look at Cerwin Vega.  Raw, uncouth, unrefined, but excellent for loud raucous music.

MoR option would be a pair of JBL.  Many choices for less than $500.

Polk can be hit or miss.

If your tastes are more demanding and you listen to acoustic jazz, classical, etc in addition to Rock, then look for a pair of Triangle Titus 202.

Moderate sized boxes, 91db efficient, benign crossover load, and can play loud when connected to low power tubes (in fact they are an EXCELLENT match with tubes).  Outstanding transient response, and reproduction of instruments.  Soundstage space in spades.   Can sound bright, and no bass below about 75 hz (but will couple well to a sub).  

You have a lot of choices.

Wharfedale 10.1 or 225 models are pretty decent and much better than their price might suggest.

I had the Diamond 225 in use for the past 3+ weeks. The sound was quite good and I could listen for hours to their comfortable sound. However, I wanted something a little more when it came to the top end detail and opted for the Elac B6.2 speakers instead for $170 - locally, and still in the box.  I've read about some crossover upgrades and would gladly spend $100 in components to slightly enhance the sound.

I have a good friend who owns a pair of  Roger's Studio 1a that are in immaculate condition placed on 14" stands I believe, not positive on that but they  sound pretty fantastic in his room. Still impressive after all these years. A larger bookshelf than may be desired though. Finding a pair of them in that condition would be challenging. Another classic sounding bookshelf would be a pair of Snell type Jll's.   It will come down I think on the type of Music you fancy most. 

Any PARADIGM STUDIO 20 pair in good shape would be awesome. I have a pair of Klipsch RP-160M speakers ans do not find them too bright for my Classical listening. The Paradigms are a step up.