best matching full range speakers for sx880 pioneer (60W rms per channel)

For $300 some used Cerwin Vegas. That would do the trick if you want full range sound. They won't be the most refined things around but they sound pretty good, fun, and probably the closest to full range you are going to get in the $300 range. 

The large Advent would be a nice fit. Plenty on Ebay within your budget!

I have the SX-680 version (45 wpc) and have had it paired with both my JBL L40 and ADS L520 speakers for many many years. I slightly prefer the ADS sound for its less fatiguing tone but the JBLs sound more dynamic. These receivers deliver surprisingly good balanced sound. You just need to decide what kind of sound you're looking for in a speaker. There's an ADS L810 listed right now for $275 and could sound real nice with your SX-880. Great speakers with particularly solid bass.

Had a 50 wpc pioneer of that vintage (SX-950 TW) or something like that back in day.  Large Advents were the go to at that time. 

Nostalgic is nice and dreamy while thinking
Performance and sophistication is preferable when listening
 you make the choice.
If it matters a 30 year old pair of properly set up Vandersteens 2Cis about 500 bucks will smoke all your dreams into a great listening expierience.
Best ,
JohnnyR