bass cab with parts that I have
 

Let me start by apologizing about beating this dead horse of a topic.

I have two 15's that are from an old car stereo system I had and I want to build a bass cab out of them. They are in a box already. These are their specs:
Size: 15"
Magnet: 100 oz
Power handling: 600 watts RMS/800 peak
#v.coils/Impedance: dual 4 ohm v.c.
Freq. Resp.: 18-2500Hz
Efficiency: 87dB (1W @ 1M)
Fs/Hz: 25
Re: 6.8
Qts: 0.58
Vas/liter/ft3: 145.9/5.15
Xmax: 13.7
Depth: 7-1/8"

I have and ampeg svt6pro for my amp with these specs:
RMS Power Output (8 Ohms): 750-Watts
RMS Power Output (4 Ohms): 1100-Watts
Preamp: Tubes (2 x 12AX7)
Driver Tubes: 2 x 12AU7, 2 x 12AX7
Power Amp: Solid State
Tone Controls: Bass and Treble
Mid Tone Control: 4-Band Semi-Parametric
Ultra Low/High Boosts: Yes
Balanced Line Out: Yes, Pre/Post with Level and Ground Lift
Effects Loop: Yes
Preamp Out/Power Amp In: Yes
Dimensions (W x H x D inches): 19 x 3.5 x 15
Weight: 41 Pounds

From what I have gathered from all the research I have done is that this is a hard task. I think it will work if I get some speakers for mids and highs and use a crossover of some sort to separate out the different frequencies for the different speakers. Any suggestions/concerns/advice is greatly appreciated.

Those a car subwoofers and they are not very usefull to reproduce the sound of a bassguitar. These kind of drivers can shove some air, but they lack the efficiency in the upperbass and mids.
These drivers are build with a totally different goal in mind.

Sorry, but you should sell them or if you keep 'm then put them back in a car (or in a hometheater setup).

This is a car sub with very low efficiency! you will find it unsuitable for your needs pretty much whatever they are but don't let that stop you
running it via a separate power amp and cross it out actively
below 300Hz it will sound bad if you go higher and find a mid box of some kind.:bassist:

I wouldn't even use those any higher then 80hz. Add a pair of 12" PA/bassguitar drivers that handle it from 80hz and up. Then indeed use an active crossover and a seperate poweramp (or a big subwoofer plateamp) to power and filter the 15" subs. That could make sense (if you play without a PA)

Those speakers are seriously not suited for a bass cab. But you already did realize that. What I can think of is building a small closed cab for them, to limit bass response and excursion. Another option maybe a small ported cab with a serious highpass filter. You might combine the two 15's with one eminence alpha 6a.

This right here is what is going to kill your dream.

IF (and thats a big if) those driver actually do handle 600 watts, their max SPL could be matched with this Legend B102 and ~150 watts. Load 4 of those legends into two vertical 210's and you could match the same volume with ~50 watts. Or with the same 600 watts of power you would have a gain of about 12 db. A 12 db increase is ridiculous, its like going from 100 watts to 1000.

Thats not really true.
You can't compare apples with oranges......
That 15" carsub is an xmax monster that is only build to shove air in large quantities. The B102 is to reproduce the frequencyrange of the bassguitar efficiently.
If I have time I can do some (max spl)simulations of that 15" carsub compared to the B102.

Well thanks for everyone's help and opinions. I think it is possible to create something with a few more speakers and a crossover that might work, I think I might take the time and money and put it toward a new cab of some sort.

That's probably the best solution.

As you have probably gathered, these drivers are not really optimum for high spl bass cab use. This is due primarily to their relatively low sensitivity. However they can be used. They will make bass. Use the box they are currently in, but wire the dual 4 ohm voice coils in parallel (results in a 2 ohm driver), than wire the 2 drivers in series to get a total impedance of 4 ohms for the cab. This will allow maximum power output from your amplifier. In use, orient the cab so that the drivers are stacked vertically to improve dispersion, and allow you to hear your own playing better on stage.

The drivers are rated to 2.5KHz, similar to many single driver and non-tweeter bass cabs. There is no harm in trying them as-is. If you want to add a driver for improved dispersion and extension in the HF, add a single high efficiency pro driver midrange with a 2nd order, 4 ohm crossover at around 1 kHz. An Eminence Alpha 6 (4 ohm) should work fine.

Keep in mind that there will be spl limits in spite of the high Xmax of the drivers. All drivers exhibit thermal compression. This is rarely talked about and even more rarely included in driver specs. Basically, even high sensitivity pro drivers are no more than 5% efficient. The remaining power (95+%) is turned into heat. The heat increases voice coil resistance. Good pro drivers have 2-3db of compression at only 100w. It gets worse as power increases. Your 1100w amp would seem to be adequate, but due to thermal compression it will see a higher impedance load. During a gig you could end up with only a few hundred effective watts. Further, the higher vc resistance increases Qts. Drivers starting with a fairly high Qts (like these) become poorly damped, and the sound becomes muddy and has poor note definition. Sealed and ported alignments drift as Qts rises altering the response profile.

Bass guitar use really is a lot more demanding than car stereo use. It really is best to start out with purpose intended drivers. If cost is an issue, use of what you have available is the best you can do. Otherwise, drivers designed for bass guitar or PA use will provide much better results in smaller, lighter cabs.

Good luck.