Should I try to improve speakers at $2000.00 range

Zoot45,
I can only add a couple of comments. You really need to figure out yourself, what you want different in a speaker. Certainly things have changed, that doesn't mean you have to. If you can say, I really want more detail or I really want a better sound stage or midrandge is too recessed or too forward, top end isn't smooth, you can then get firm recommendations. Otherwise "can you get better speakers today for $2000?" the answer is clearly yes, but will they make the difference of what you are after? Only you can answer that. I don't know where you live, but my guess is that some audiophiles out there would let you visit and make comparisons with your Celestions. If you don't know what you want differently, you need to listen for yourself and decide what you would like to hear differently. Everyone here could then chime in with clear examples of how different speakers sound differently from your SL700SE

Marty, the potential for greater frequency extension was among the first thoughts to enter my mind when I saw that the Celestion speaker in question was a smallish bookshelf. It's not only a matter of build quality or house sound; it's a matter of absolute sonic potential - and $2K can buy you a lot of sonic potential, especially if one buys previously owned speakers!

I am absolutely not attempting to belittle or marginalize those who have a limited budget for audio! My principle holds true whether considering a low cost or high cost system. Technology changes and assures that better sound is available at all price points as time goes on. i.e. I would much rather have my $300 Denon receiver in my living room system than a 20 year old receiver for $200! I had a vintage Sansui receiver; no way would I prefer it over the Denon. The Sansui was sloppier sounding with poorer transients. You don't have to spend inordinate amounts to take advantage of the more recent developments in technology. :)

2000$ is a VERY busy place in the world of speakers. Given the age of the celestions and the advances in materials, modeling and manufacturing, I'd see NO reason the amount of $$ mentioned in the post should not easily better the Celestions.
The rest of the system is somewhat better than the speakers, so I'd go for a listen. Couldn't hurt, now could it?

The best advice is to go out and hear as many speakers as you can. Hear stuff above and below your budget, so you can determine what the relative value of things are. If you hear something that really appeals to you, see if you can bring it home for an audition. If you can't bring it home, see if the dealer will let you bring in your speakers to compare. If they won't let you do either, find them here and compare at home. If you pay the right price, you can sell them for little to no loss if they don't measure up to what you've got.

That's all a bit of work, but consider it a labor of love. If you don't hear anything that makes you want to change the speakers, you'll be that much happier listening to music with what you've got. It's a win-win IMO.

We can all recommend stuff for days. We can recommend what we own and/or would buy in your situation. At the end of the day, it's just food for thought, nothing more, nothing less. Nothing beats hearing this stuff for yourself. I own Audio Physic Yara Evolution bookshelves. To me, they're absolutely perfect in my system for my budget and quite a bit more. I came to that conclusion by listening to a ton of stuff. Doesn't mean you or anyone else would agree after hearing them.

I haven't heard Celestions, but everything I've heard about them has been very positive. Trying to improve them may be futile or fruitful. Either way, you'll have an answer that's certain to you.