For midrange, how about the ACCUTON C²79-6 ceramic driver. Covers 200Hz-4kHz and is intended for 1.5L-4.0L sealed enclosures. Low resonance frequency allows first order filtering from 200 to 4000 Hz.

Or, the Monacor SPH-135KEP. It's a 5.25" driver, but is ruler-flat from 120Hz-5kHz w/ an Xmax of 3.25mm

It's smaller brother, the 5" Monacor SPH-102KEP has nearly as deep LF, but has a rising response above 1.5kHz. It has a much lower Xmax of 1mm, so lows might be a problem. Though, it is intended for a 3.7L sealed box.

Audio Technology (Skaaning) 4" Flex Unit 4 H 52 06 13. It's got a 4mm Xmax and a pretty flat from 100Hz-4.5kHz.

Peerless 4"(830872) High Definition Sound (HDS) NOMEX. FS 85Hz and 2.5mm Xmax.

The Peerless 832873 goes a little deeper w/ an FS 58.6Hz and Xmax of 3.5mm