If the system has a vintage vibe and aesthetics matter for the purpose of matching, the REL Classic 98 or 99 may fit the bill. Unsure what veneer your Heresy’s are (Birch, maybe?) since Klipsch appeared to offer multiple options for the first generation of Heresy. The REL Classic line is in Walnut.
The only issue is no continuous phase adjustment. Otherwise, I’ve found the Classic to be a little more rounded than prior subwoofers (HSU ULS 15, Rythmik F12G, Rythmik one-off by Omega Speaker Systems), but easier to integrate which I feel may be due to usage of the SpeakOn cable that comes standard.
FYI: I’ve never listened to a pair of Heresy’s so keep that in mind with my purely aesthetic suggestion.