When to repair or replace speakers?

So here is the situation folks, I have a pair of Polk RT2000P speakers that I have been using for 9-10 years (bought used from this site so maybe 15-years old). Recently I hear a thumping / knocking sound of the left speaker sub when under normal listening levels when heavy bass is present in a movie. I think the surround may have given out allowing one of the powered 8" drivers to bottom out but I am not sure. It has happened a few times so I don't think it is a fluke but i dont see sny physical damage to the cones or the surrounds..i am considering getting them re-coned but since there are two drivers per cabinet and two cabinets, I am looking at 4 cones and I am not sure that this will be cost effective. This also assumes that I have ID'd the problem correctly and that it is not power related with the built in amp throwing too much juice.

That said, I know that at one time these speakers were around $1k each and well regarded. I have to assume that to replace with something new and equal or better quality I am in the same price range (maybe more if I need to buy a separate sub). If I look for replacement / upgrades what price range am I in now and what do you recommend?

How much should I expect for the repair bill on them with the issue I described?

I am running an Integra Audio DTR 40.1 reciever and I use this setup for movies and CD listening through a Marantz CC4003 changer with a Polk CS400 center speaker.
cones more likely are OK especially if OK visually. voice coils might be under jeopardy. when your amp fries you'd definitely know or you can try to find matching complete drivers.
Did you check the sccrews holding the woofers? just see if they are tight?
They may be all that is wrong.
Also if you were going to replace the cone forget it. Just buy all new woofers.
You could ask over at a place like Madisound. They sell replacement speakers and a lot of DIY kits. The guys workingthere could suggest four decent (read not too expensive) replacement driver which would be good.
A bad power supply filter cap(power amp) sometimes results in the same symptoms. That can sound exactly like a voicecoil bottoming out. If you can get to the built-in amps: look for signs of leakage around the electrolytic caps.
Thanks for suggesting some basic things that I can look at first. I will have a look this weekend and see if I find anything.
I have not looked at the filter cap yet but last night I had a look at the woofers.

If I tap hard with my finger on either of the woofers in the suspect cabinet I can create the noise that I am hearing. When I slow down and push gently it seems that the sound is created by a rubbing / knocking on the side of something. If the woofer is pushed in anything less than dead square so perhaps loose / worn surrounds are allowing it to respond and get off axis or is this a symptom of a signal issue causing a funny response?

i will check the electronics next
It's possible that the voicecoil(or former) is rubbing the magnet plate or pole piece. It's important, when trying to determine if rubbing is occuring, to insure an aligned movement of the voicecoil in the gap. The driver should be out of the cabinet, on a flat surface(magnet down). The cone should be manipulated(gently and evenly) from exactly opposite sides(180 degrees) of the cone's edge. You should be able to tell(visually) if the spider(suspension at the narrowest point of the cone, or glued to the voicecoil) is deformed at all. If the foam or rubber at the outer circumference of the cone is not deteriorated or torn; it's still good.
I would contact Millersound http://www.millersound.net/about.htm, who is THE artist in this realm. Please don't hold me to this, but figure on the cost coming in under $200, perhaps much below.
IF your drivers are the problem; I second contacting Bill at Millersound. He'll insure they are as close to their original parameters as possible, when rebuilt(very important, performance-wise).
Rodman99999, "IF your drivers are the problem."

Rebuilding drivers shows less than 1% of Bill's abilities, though obviously represents the lion's share of his business. He'll make most any repair a loudspeaker requires.

For a bit of background, he does things you'd never expect paints cars, engine/transmission rebuilds, turntable/electronic rebuilds, lung transplants(just kidding on that last one), all at expert level. See that perfection itself in his Packard he painted himself http://www.6moons.com/industryfeatures/forbidden/forbidden_10.html
A very talented man, of many passions. Most of which; I share(though I'm forced to pursue them on a truncated shoestring). I haven't spoken to him a great deal, but have always enjoyed the conversations.