Many people take it as a given that positive experiences have intrinsic value, but I don't think the matter is as clear-cut. So here I present a simple experiment which you can try if you are in the mood for listening to music.
Experiment: Listen to some music you like. After a while, mute the music. Alternate between silence and music while trying to appreciate how each feels.
When I mute the music, there is a lingering freedom from what I like to call the "heaviness of being"—basically any pain, unease or listlessness that I may otherwise feel. Turning the music back on definitely adds an enjoyable or gripping quality to the experience. But when I mute it, I don't really get the sense that the experience has become worse, and yet I don't find the peaceful silence better than neutral. This would seem to imply that listening to music is at best neutral.
To be clear, experiences can be impactful and leave a memory associated with meaning and value. Some event can cause me to feel deep sadness and still have this effect, even though I would hardly consider the moment-to-moment experience positive. And my aim here was precisely to call into question the intrinsic value of pleasure, its value when experienced for its own sake. I am curious about your thoughts if you tried the experiment!
Cannot moral realism be grounded at least in suffering, though? It seems inescapable to me that generating suffering in an experience machine would be disvaluable. For the experience to be suffering, it may require a component of wanting it to end, but this would still be a felt quality, right? So no matter when or where the suffering was experienced, no matter "who" experienced it, it would still be disvaluable due to its inherent nature.