I'm fortunate enough to have a pair of SL3's I bought in 2002 basically new. Later that year due to some electrical faults in my new home, I ended up getting new panels, power supplies, crossovers, etc., all funded by the idiots who installed the electrical system in my home! I felt nothing, BTW - those bastards could quite easily have killed my if I had even changed out a switch before the issues manifested. Turned out the old panels were fine, so I left them in (even tho' replacements only cost ~$500 at the time)... Not sure what storage in the original box/plastic packaging might've done to them in 20 years, but I still have the unsused replacement panels!
I think if I were in your situation, I would replace the panels, and maybe follow
@Brandon Hartwick's thread (
How to: active bass section) on bypassing crossovers and using an active system.
Reason I say this is:
1) if you decided to go with Summits, you'd end up having to replace the panels sooner-or later, so add #3,000 USD or more to the $5,000 price for used Summits...
2) To replace with current (equivalent panel area) speakers (allowing for gains in efficiencies), that's still the 13's @ ~$15,000
3) Panels replacement is $3,000. Amps like Brandon used for Bass would cost you about $1,000 for BOTH channels. In this model you wouldn'y have to buy expensive Crossover componenets and futz around with tuning them, either.
Lastly, maybe consider adding a (musical) subwoofer to your system ($1,000 - $2,000).
Just my $0.02. Enjoy whichever path you choose!