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

Polk makes a $300 speaker with a 6-1/2" mica-reinforced poly driver
ELAC has one with a 6-1/2" aramid-fiber driver
- either should be pretty stiff

also, see what JBL, Epos & Cambridge Audio have

you can do pretty well these days for low bucks, but you will give up SQ in the mid-range to get the low freq.s

or call up Andrew Jones and tell him you've got another challenge for him...

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.