overpowering and loud listening.

The extra wattage is a good thing in that his amps would be less likely to clip. The constant playing at loud levels should have caused soime audible damage if it was going to. Since it didn't cause anything obvious to breakthey may be OK. BUT listen to the speakers very carefully for rubs and or missing sound, the damage may be there even though it is not immediately obvious. So there is a downside, perhaps you can have them assessed by a pro speaker tech for problems.
It kinda sounds like you don't feel comfortable with this deal so why do it? You should be happy with the gear you want to buy.

My concern is they have a manufacturing recommended power rating of 300 w and the owner was using Pass x 6oo's to power them.

given that the manuf has a higher power rating for them & that the owner used X600 to power them is a very good thing (as Mechans has already written) as this means that the amps most likely did not clip the speakers. Clipping does more damage than playing loud/very loud as the distortion created by clipping gets past the x-over ckt & destroys the drivers - tweeters first. Using an under-powered amp would have been the worse option.
And, it looks like the Andra2 like to be played loud guessing from the manuf's rating.

I recently auditioned used eggleston works andra 2 speakers

did you take your own music to the audition so that you could listen attentively to passages in your music that you now know very well? If not, you should have! The whole point of auditioning is to see that the new potential buy is better than what you own + that the speakers are healthy as far as you could tell.
Speaker drivers are very hardy & quite tolerant of abuse as long as the amps don't clip for long periods or often. If the drivers are getting abused frequently then the drivers are likely to show this when you listen - uneven SPL from left/right which will move the soundstage right/left, buzzing from the cone driver & maybe some physical damage on the driver upon close-up inspection are some of the tell-tale signs.

Simply playing a speaker loud is not sufficient grounds to reject buying it. If it is, then, cough up the money & go buy a brand new speaker & put your own volume/SPL signature on it. Speakers are meant to be played loud esp. if you get into your music & are having a good time listening.... According to me, no point in buying an Andra2 pair of speakers & then playing it at 70dB SPL! I want that room to rock when the Andra2 are playing....

hope that this helps.

speakers had seen a normal usage and they've played music. they will sound better than new ones.
i prefer to purchase car that had been a daily highway driver i.e. not driven slow. almost same applies to speakers.