Hi dunbroke,
Pillows are a very personal item and while there are a few “needs” involved because they are an important part of keeping your upper spine, neck, and head in alignment … beyond this they are very much a matter of personal preference. The pillow thread here has some good information and links to some good resources which can provide some guidelines and options for pillows but beyond the basic “needs” they are mostly a matter of personal preference.
Side sleepers need thicker pillows than back sleepers or stomach sleepers (which generally do best with thin pillows or none at all) … but the thickness a side sleeper needs also depends on how far you sink into the mattress (which affects the space between the head and the mattress) and on body type and size as well so yours are probably thicker than you need … and possibly firmer than you prefer.
A sore neck/back can be a symptom of misalignment but the “fix” depends on where the soreness is. If it’s just in the upper body … then changing the pillow can often resolve the problem. A second issue could be if your shoulders aren’t sinking in quite far enough but the aspen is fairly soft so this would be less likely (depending on your height/weight and body type). A more specific set of “symptoms” and their location(s) may help point to the underlying cause of the soreness. Some extra softness may help if the issue is that your shoulders aren’t sinking in far enough but it sounds like it could be more of a pillow issue.
Phoenix