Hi Bill,
I have seen you respond to many questions posed by others and do recognize that you really know a lot!
My problem is that I tend to make things that are simple complicated and also not provide enough background information or write so poorly that I end up wasting everyone’s time……Interspersed within your response, following the # sign, see my counter-comments 
There's a couple of problems with what you want to do here.
You state that your goal is to widen your AM response. That being the case, filter #475 is not going to help you at all. In fact it's going to make things worse.
# My goal is have a reasonably wide SSB TX pass-band, "only" around 4 kHz+
) Also, I am feeding audio in through the front mic-jack with known limitations associated with this approach. When playing audio and striving to get everything dialed in, I have my TS870 on to monitor my audio. I am using a condenser mic that appears to fit my natural speaking voice well and Behringer outboard gear with a Jensen Audio Isolation transformer (their PI-XX box with the transformer in it) as the last piece of hardware just before the mic jack. I am feeding audio in a balanced manner into the front (I am using star quad cables) from the Audio OUT of the PI-XX.
An AM signal is composed of three pieces: the carrier, an upper sideband and a lower sideband. The upper and lower sidebands are basically mirror image copies of each other. This means you're transmitting the same audio information twice.
This means that for AM, you need a filter that is twice as wide as your desired audio bandwidth. If the YK-88A is 6KHz wide, that means you get a total of 3KHz of audio bandwidth. The Inrad model #475 is only 4KHz wide, which means you'd be limited to only 2KHz of audio bandwidth. This is why they labeled it as a narrow-band replacement for the YK-88A.
Furthermore, there's a few other factors that come into play here. Remember that the TS-950SDX uses the DSP unit for both SSB and AM modulation. By default, the DSP will limit you to 3.1KHz of TX bandwidth on AM. If you use the power-on mod to enable 6KHz of bandwidth, that will only work on SSB unless you have a specially patched firmware. (There's an article in the modifications section which describes it.)
# I am able to switch both the HPF and LPF OFF on SSB and AM.
You said you have the latest firmware. You did not say if you had the patched firmware with the HiFi AM mod. There used to be a guy in eBay selling ready made EPROM chips that you could use to upgrade, but I don't think he's doing it anymore. I don't know if you obtained a chip or burned one yourself. If the latter, you can download the modified EPROM image from here:
http://people.freebsd.org/~wpaul/ts-950 ... patch.html
# I am awaiting information on the latest patch that the previous owner of the rig had done. I believe it is the one to which you inserted a link to.
Also, you can use the service menu to allow you to change both the 1st and 2nd IF filter settings for TX as well as RX. This includes setting the 2nd IF filter (8.83Mhz) to bypass, where the filtering is completely disabled. This in conjunction with the firmware mod above will give you the widest possible TX bandwidth (6KHz, basically the same as the radio's eSSB bandwidth).
# Oh, I didn’t think of the 2nd IF filter bypass. I would imagine that I can get to the right MENU by turning on the POWER while pressing the ENT key as per the Manual?
If you download the firmware patch file at the bottom of the page mentioned above, it contains a FAQ.txt file which explains how to change the service menu setting and enable the "off" setting in the LPF menu (for full HiFi mode).
# Appreciate the FAQ.txt file information 
Lastly, note that the 1st IF (455KHz) filter setting doesn't have a bypass (all off) position like the 2nd IF setting does. This setting actually has no effect on AM transmit, but it does affect AM receive. You can set it to 12Khz, which is not bad, but you can also easily create a bypass setting, as long as you don't mind giving up one of the extra narrow CW filter slots. Basically, you can connect a jumper wire in the place where the optional CW filter would go and just pretend it's a bypass setting. This will give you a little better bandwidth on RX. If you don't have the optional extra narrow CW filter in your rig, the slot is just sitting there unused anyway.
# I am thinking of replacing the 1.8kHz filter (I think it is the YK-88SN-1) at the 2nd IF with the Inrad 4 k filter.
# Oh, Inrad sent a one-pager about the install that I found confusing. It talks about cutting off "the last two pins flush to the white block" and also "removing small PC assembly." To remove the "PC assembly" I would need to unsolder four pads. Also, the picture of the filter doesn't jive with where those two pins that are supposed to be cut off sit?
# Let me know if what I am proposing above makes any sense?
Cheers!
JJ