Recently purchased a Sherwood s-7025 receiver and one of my speakers sounds muffled.?

Try a different cartridge next. Always good to have an extra. The wires that connect to the cartridge were notorious for breaking without knowing, so check. I think the receiver is fine. Just an old timer that grew up on turntables.

get tuner cleaner w the red spray concentrator,remove control dials,,you might have to remove the face/dial vernier  panel,spray into the pots ( potentiometer),check speaker fuses inside the chassis,as the guy below said,get the balance turnable switch,treble bass,midrange,function dial or switch,, blow out all dust,, either use canned CO2 to do this,or a small electric toy inflator using the nozzle to blow the dirt out

OK,from your description, it sounds like you already tested your speakers thoroughly.  So it might be a problem in your Sherwood receiver.     Do you have a headphone jack?  Plug in a pair of headphones.  Does one channel sound muffled when listening thru phones?  Also, can you connect your turntable up to another amplifier or receiver and try that configuration out?  Do you still get the same weirdness / muffled sound on one channel with the other receiver / amp?    By swapping out the Sherwood, that will tell you if the Sherwood S-7025 is faulty. 

If it only does that while playing vinyl and not with your CD player or radio then it could be the record player...Hook up an FM antenna and listen to a clear FM station near by type of music doesn't matter what matters is its a quality station that cares..If the problem persists then its the right channel of the receiver...Get some Creig D5 Deoxit and spray it into the balance control...You may have to take the top off in order to get in there from behind. Also if you do take the top off place a towel over the inside components first so that over spray doesn't get into other areas of the receiver, spray a little and work the knob back and forth spray some more and work the knob again all the way from side to side...If not and its your record player it may be that your cartridge in not aligned properly...Get yourself an alignment protractor. Bearwald Heybrook Stevenson, doesn't matter...A lot of that comes down to personal preference and what works best for your particular turntable and tone arm....Watch a couple of videos on you tube on how to do it and what needs to be done.

The balance control switch is dirty or possibly worn out.

https://alexav.com/cleaning-vintage-stereo-receive...

https://www.youtube.com/watch?v=ukA8Qy-M4fY

Replace the stylus and cartridge. 

Noise or crackling as you turn a control is not unusual, it's just dirt or wear in that control.

Sometimes just rotating them end to end a few times will clear it; otherwise they need cleaning or replacing (or just put up with it).

Also, find every switch or selector of any type on the amp and operate each a dozen or so times - the same thing can happen with those, tarnish from lack of use causing odd problems, but those normally "self clean" with a bit of use.

Just something as simple as that may clear a minor fault.

If it's still present: You seem to have already proved that the speakers are OK.

Can you swap left and right from the turntable? If you do that and the fault is still worse with the amp set to its right side, then the amp is faulty.

If the problem moves to the left with the inputs swapped, it's the turntable cartridge or wiring - or just possibly a bent stylus that is near the limit of its travel in one axis.