I hear you, but am locked into a cabinet design - the original - built MAYBE before T/S parameters were created
Correct.
I personally have not had the ability to calculate the port frequency of the earlier classic porta flex cabinets.
As far as volume, that is easily done. And the volume is suitable for most 15" drivers. Far as I can tell it's a realatively small volume. Which in general gives you more power handling.
Usually the issue would be, some drivers tend to prefer larger volumes. And if put in a smaller box they tend to be boomy. Or there is a big response peak in some frequencies around 60 to 100 hz. So some describe certain notes on the bass
" blooming"
According to modeling. Even a delta and beta need larger airspace and would bloom and boom in a typical early porta flex. But it seems to not even be a complaint with deltas or betas.
You should get at least 50 to 80 watts into a delta before it started farting out. With the typical 3mm to 4mm xmax.
But here we are. It's farting out with lower wattage.
I believe but can't confirm these earlier boxes are possible grossly high tuned around 60 to 80 hz. So anything below that port tuning would cause massive come excursion, or farting out. It's just how a reflex or ported box works.
So even standard E or A could push a speaker pretty far especially drop D
There is very few speakers that would work in a high tuned box. But I know a
D-130 in a modeling alignment shows a ridiculous high tune around 75hz for normal common Butterworth alignments.
Even JBL didn't use such absurd high tune. But mechanical wise to match the speaker to a alighnments you'd never use. It is high around 75hz.
Likewise the EV15L would not align to such a high tune. But it behaves somewhat similar to the JBL
This doesn't mean they wouldn't fart out. High tune is high tune.
In general my gut tells me a JBL D-130 K-140 or EV15L or Even the higher Fs Black Widows designed for Guitar. Would possibly sound better in a higher tuned cab. Could possibly be less or same fart factors.
It be nice to actually know what the cabinet tune is. Likewise difficult to recommend blocking some of the cutouts. This would lower the tuning.
Far as drivers that won't bloom in smaller boxes. EV15L , D130 or a Eminence Neo
Deltalite II 2515 is a 15" that does well in a smaller box and is a 5mm driver for Xmax. And in theory should not fart as soon as the 3mm delta.
Also would work in a sealed box. Not sure if anyone has tried sealing the ports completely on the older cabs. Or attempt to lower tuning by blocking a few ports.
The Black Widow 1501DT and 1502DT are famous for pedal steel guitars and pa systems not JBL copies but follow the design concept. Probably very good candidates for tone and power handling.
Replacing them with the older tubes you have is going to be a big help. Knowing that, I'd go back and try that Delta again once you get your amp fixed and good tubes put in.
