Perhaps a send/return module like the knob.farm Ferry (4hp, $140) and a really good external pedal? If reverb is really important to you, I don't think in-rack effects match the quality of external ones yet, with the possible exception of the Strymon modules. If you have anything besides modular that you might also want to use effects with (other fixed-architecture synths, guitar, etc.) that tips the balance further.
-- plragde
Can this be done with the Doepfer A-138o? I have only used it as an aux for modules in the past.