Can't find a series matching center speaker(no longer making)

Maybe try looking for a used one on ebay, Craigslist, audiogon, etc. or move the L/R speakers to stereo duty and buy a new front trio. That's what I would do. Unless the front L/R are that good, then keep searching for a used one to come up for sale. What are they, anyway?

You can use a different series made by the same manufacturer that uses the same tweeter as the ones you have.

Ideally you need the JBL EC35 or EC25 but these also should work great with the E100s:

JBL Voice

JBL BalBoa Center

JBL N Center

Also, check eBay for better prices.

I forgot to mention, the JBL Balboa 10's are good surround speakers to use with the E100's.

The E30s and E50's are much better speakers but are hard to find these days. If you could get them, they should be the better option.

There is a huge difference b/w the 25 and 35. The EC35 has larger mid bass drivers and a midrange. It sounds so much better than the EC25. I bought 2 EC35's a few yrs ago and I love them so much. I gave the EC25 to a relative after I bought and listened to the EC35. That's the reason I bought 2 EC35's before they completely disappeared.

You might find a used EC35 on eBay.

One thing I can think of in place of the EC35 is using an E30 or E50 as a center speaker.

If you can get the E50's, you can use one of the E50's as the center speaker, which would be the best match for your front E100's. The subwoofer does not need to be from the same series, you can get any sub you like.

Edit: There is a NIB pair of the E50s on eBay now. If the price is OK to you, get them ASAP.

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Check out the BIC PL-200 and the Polk Audio PSW505. They're great subs that don't break your bank.