Coliseum Lead+Bxr Spectrum

[FeeltheSunn]

Hello all, I wanted to ask you all if you had speaker/hardware recommendations for a lil project I have. So I have an ealy 70s Sunn Coliseum Lead head and 2 fender bxr spectrum cabs(2x10+1?18) I want to run it through,problem is both cabs were are at less than 100%,one cab needs the 18" reconed and the other has had the crossover removed and was rewired.thinking about kkeping the shells and installing all new speakers+crossover specifically to match the frequency range/power rating of the head. I hear many of you used to work for Sunn what 18" and 10" speakers would be best for this head?

[EdBass]

I think you would be best off adding another amp, and running the 10's and 18's separately, most efficiently with an active crossover. If you could find a Coliseum slave, that would be pretty bitchin', 
I couldn't find a lot of info about the Spectrum cabs, other than they ran 8 ohm 10's in parallel and a 4 ohm 18 for a nominal rating of 2 ohms. Not sure what the xover was or what impedance impact it had, but running two of those cabs paralleled would be showing a single amp a load around 1 ohm, which could turn your old Coliseum into a short lived space heater.
Those cabs were made to run with the Fender BXR Dual Bass 400 amp, which was actually two 200 watt power amps with a built in active xover, that could be bridged into a 400 watt mono configuration. The BRX Spectrum cab had three inputs, high, low and full range using a passive crossover. I'd guess the passive xover was 500-800hz, but have no idea about the actual specs.
You can get high powered xovers through retailers like Parts Express;

https://www.parts-express.com/search.aspx?N=4294967118+4294964742&Nrs=collection%28%29%2Frecord%5Bendeca%3Amatches%28.%2C%22P_PortalID%22%2C%221%22%29+and+endeca%3Amatches%28.%2C%22P_Searchable%22%2C%221%22%29%5D&Ntt=crossovers*&PortalID=1

They are knowledgeable folks that can guide you where you need to go.
As far as speakers, I'd look at something with fairly high sensitivity, and as your amp is only 300 watts into 2 ohms, 200 watts into 4 ohms, and 125 watts into 8 ohms, no need to drop big coin on 500 watt woofers.

Can't say for certain, but I don't think any ex-Sunn employees have posted here in several years. 

[FeeltheSunn]

I think you would be best off adding another amp, and running the 10's and 18's separately, most efficiently with an active crossover. If you could find a Coliseum slave, that would be pretty bitchin', 
I couldn't find a lot of info about the Spectrum cabs, other than they ran 8 ohm 10's in parallel and a 4 ohm 18 for a nominal rating of 2 ohms. Not sure what the xover was or what impedance impact it had, but running two of those cabs paralleled would be showing a single amp a load around 1 ohm, which could turn your old Coliseum into a short lived space heater.
Those cabs were made to run with the Fender BXR Dual Bass 400 amp, which was actually two 200 watt power amps with a built in active xover, that could be bridged into a 400 watt mono configuration. The BRX Spectrum cab had three inputs, high, low and full range using a passive crossover. I'd guess the passive xover was 500-800hz, but have no idea about the actual specs.
You can get high powered xovers through retailers like Parts Express;

https://www.parts-express.com/search.aspx?N=4294967118+4294964742&Nrs=collection%28%29%2Frecord%5Bendeca%3Amatches%28.%2C%22P_PortalID%22%2C%221%22%29+and+endeca%3Amatches%28.%2C%22P_Searchable%22%2C%221%22%29%5D&Ntt=crossovers*&PortalID=1

They are knowledgeable folks that can guide you where you need to go.
As far as speakers, I'd look at something with fairly high sensitivity, and as your amp is only 300 watts into 2 ohms, 200 watts into 4 ohms, and 125 watts into 8 ohms, no need to drop big coin on 500 watt woofers.

Can't say for certain, but I don't think any ex-Sunn employees have posted here in several years.

The bxr spectrums are designed to run 4ohms each at 400 watts

[loudthud]

A crossover will split the 18 from the two 10's so that the net impedance is 4 Ohms, not 2 Ohms like they were just in parallel.

Those BRX Spectrum cabinets probably want a crossover frequency in the 200 to 300 Hz range. Parts Express doesn't offer anything in that range except maybe a low pass only intended for sub-woofer use. You could try to copy the crossover in the one complete cabinet, but the inductors probably don't have any meaningful numbers of them. Back when catalogs were commonly printed, Parts Express had a page where you could lookup what inductors and capacitors were needed to make crossovers of various frequencies. Probably still on their website, but I couldn't find it.