Some people say the Delta Pro 12A is similar to the EVM12. I don't think it will have quite as much character but it will take more power and put out more lows. It won't take a lot of power or put out a lot of lows...it's really more of a high quality mid range driver for PA cabs IMHO.
The only way you will find out how well the Delta Pro pairs with your existing cab is to try it. Maybe it will work great, maybe it wont. If you are willing to spend 400€ to find out, go for it.
Do you like tweeters? If yes, I would suggest a second cab tuned like the first with an added tweeter an HPF section and L-pad for the tweeter.
The problem with mixing cabs:
AFAIK it's really not about driver size. Instead it's about how the cab is tuned. The port is tuned so that it extends the low end of the cab. Essentially at some frequency the driver starts rolling off, and the port starts to become increasingly resonant so it can reinforceme the low end. The frequency where the port is most resonant is labeled Fb.
There are many possible ways to tune a cab. The relationship between Fb and where the driver begins to roll of determines the tuning. This impacts the response curve and also factors such as group delay and phase shift. So if you had two cabs with identical drivers, but they are tuned drastically different, you would have the same type of phase shift and group delay variances that cause dissimilar cabs to sound bad together.
With enough expertise and effort, an engineer can choose different size drivers and make them work together, but I believe specific attention must be given to selecting drivers with compatible Thiele Small parameters. For example, the Greenboy fEARful cabs are all designed to work together, and the Mesa Subway cabs are all designed to work together. But it would probably not be ideal to a Subway cab and fEARful cab.
My experience:
Years ago I bought a GK 700RB/112 combo. The reason I bought this amp is because it was the best option I could find that would fit in the trunk of my Dodge Neon...and it barely fit. The amp pushes 225W into the internal speaker and makes 380W with an extension cab. I found that I was barely getting by on gigs with just the combo. No one was complaining, but I was having a hard time hearing myself and not having a good time. The amp seemed to have more power, but turning up just resulted in more compression, so the speaker was at it's limit.
I have some old BFI cabs loaded with EVM18Bs. The cabs are about 2 cubic feet smaller than the Electro Voice design for these drivers and tuned to about 50hz. My impression is they don't put out very much energy below 100hz, but they put out a lot of low mids. I started bringing one of the BFI cabs for loud gigs. This gave me plenty of headroom, but I would not say the sound was great. I would not say it sounded horrible either, just definitely not as good as it could have been.
The key hear is the BFI could take as much power as the internal speaker in my GK combo, so I at least the combination of mismatched cabs gave me the desired headroom. There is a good chance that a Delta Pro 12A will not take as much low frequency power as the 15, so you may not even get more headroom before the Delta Pro farts out.
I have attached the Delta Pro, cab tuning doc so you can view the capabilities of the driver with different tunings. The large vented design would probably be most appropriate for a bass speaker. Note this tuning specifies a 24db/octave HPF and the power is limited to 150W.
Good Luck!
