Elsewhere in this forum are recent posts offer to sell Monolith 3 for $1K & Request for $500.
I'm more familiar with Quests than Monoliths.
A previous poster referred to your 2nd pair as ML Request speakers. The image you posted clearly reads Quest z. This was the post-Quest, pre-ReQuest model.
The electrostatic panels will need to be replaced sooner rather than later if they haven't already been replaced. I feel fortunate to have gotten ~25 years from mine. Your pair is now ~30 years old. ML charged ~2K for replacement panels for my pair of Quests a few years back. I doubt the price has gone down since then.
You already have an approx valuation ($1500-2500). Subtract the price of replacement panels if they haven't been recently replaced & that's what the Quest z are worth.