Zaph ZRT 2.5 sealed opinions?

Scratch that, porting DID help - plugged the vents and the low end was gone. Without the vent, it's just pathetic compared to ZD5. It now goes at least as low as ZD5 and I don't hear any downsides elsewhere. It's tuned currently at 38Hz. Just a note - ZD5 is also ported at 38Hz in a 14L box which was supposed to be sealed.
Your box volume is way too small for 2 woofers vented. At 38 litres only you'll get a midbass peak then quickly dropping off. Vented enclosures need a larger enclosure than sealed. Often driver Vas * number of drivers is a good starting point. Model your tuned 38Hz in only 38 litres with 2 woofers to see what I mean
That's something I need clarification on - why is vented 65L good and 38L bad for two woofers?
The alignment of a vented enclosure is a balance of enclosure volume and Fb with the driver specs. If you undersize the volume you'll lose bass extension. In the case of the 18W-8531G00, 38 liters is a pretty good alignment for 1 woofer. Adding a second woofer to the same 38 liter enclosure causes the alignment to be undersized.

Blue is one 18W in 38L with Fb at 33Hz.
Orange is two 18W in 38L with Fb at 33Hz. This graph shows the two woofers wired in series so that the efficiency is normalized.
Why don't you model your current box volume and vent parameters so you can see for yourself? As I stated. A vented enclosure requires more volume than equivalent sealed to get a flat / extended response
When I am home I'll post a 38L two woofer vented enclosure response. How many ports did you try and what length were they?