I have downloaded the manual and studied it extensively tonight.
The news is not good. This is a complex unit. It has a very complex power supply, class D amp, proprietary bass management, crossovers, limiter and compressor to prevent woofer damage and parametric Eq.
I suspect your problem is in the power supply. It seems these gave a lot of trouble and there were lots of revisions. The latest was 2005. The service manual states that all units needing service need extensive updating. This is very complex requiring the replacement of a lot of components and extensive calibration.
Servicing this unit is not a project I would want to undertake.
Now this unit was designed and produced after Harmon bought Infinity.
I was on their site and entered your speaker and then tried to find a service center. However I drew a blank as there is no service center listed for that product. This may be because there are a lot of proprietary parts which may be NLA.
I would contact Harmon and see if they have an option for service. Expect a big bill.
Now you can not use a different sub. This is more than a sub. The high pass filter to the mid is passive second order at 150 Hz. The speaker input is fed to the amp, and there is a low pass electronic crossover at 150 Hz. The LFE subwoofer in is mixed in with that signal. This is similar to what I did with the design of my speakers. There is a high pass filter at 32 Hz and a compressor limiter to limit driver excursion and damage.
I don't think it would be realistic to reproduce that in another amp. There would be extensive custom work and design.
So currently your damaged speaker is rolling off at 150 Hz, But in addition you have lost that channel below 150 Hz. The LFE signal is mono so left and right information is in the remaining speaker over the frequency range of the LFE signal.
I'm sorry to say that realistically, if Harmon can not provide you service for that amp, then it is the end of the road for those speakers unfortunately.