RE bipolar electrolytics in the signal path, I have read those recommendations before too. At least one amp where, IIRC, all polar electrolytics (maybe just signal path, not sure) were replaced with bipolars . But in any case there were a boat load of green Nichicon ES caps in those pictures. The amp owner reported excellent results, as is most always the case when anyone does most anything. May be true, but who can really say...
There is not universal consensus on the qualities of bipolar electrolytic caps used in positions where they were not used originally. Some recommend them, some strongly dislike them. This is something that actually is a subject of debate at DIYaudio.com, which means that various people state their firm opinions and never change, and nothing gets resolved, lol. Can be interesting reading though.
General consensus on signal path caps:
(1) use film where possible. This often ends up being 4.7uf or smaller. Perhaps 10uF if there is room on the PCB. Wima MKS2 series is very popular, is readily available at Mouser, and it is(as far as I know) the smallest available film cap for this type of application. The lead spacing on most of the Wima MKS2 caps is 5mm but the leads are very short, so PCB lead spacing matters. There are some Wimas with 2.5mm lead spacing, not sure at the moment if those are MKS2 or a different series name. These are polyester, not polypropylene (which would be larger), but any film is better than any electrolytic. And all film is bipolar
(2a) if film is too large physically or the needed uF values are too large, then "audio grade" caps are next choice. Elna Silmic II is the most popular and is my personal favorite. Nichicon KZ, FG, KW and maybe a couple others are similar and also well liked. As with anything, there are some (a minority) who will say that "audio grade" is nothing but marketing hype and who would use standard cap grades in signal path.
(2b) If the original signal path cap was a low leakage type, then Nichicon KL caps would be used by most, but not all, instead of audio grade electrolytic caps (with film still being 1st choice). There are different indicators of an original low leakage cap, the most common one being an orange colored sleeve. I would have to search some notes to tell you other colors or other indicators (i.e. "LL" in the parts listing or schematic, or "low noise" ...there are others)
Again, above is pretty much the consensus for signal path. IMHO better to stick with what is known than the relatively seldom used approach of bipolar electrolytic. I am curious, and open minded, about the use of bipolar electrolytics, and would enjoy reading any information that you have come across (links?). But what is stated above is what the vast majority of people do. I really like the Elna Silmic II because I find their build quality to be a notch above (same thing can be said of KZ as well).
For non-signal path stuff, my favorite is Panasonic FC, which is also very popular among AK recappers in general. Other popular caps are Panasonic FM and FR, and Nichicon PW, HE, PM. All of these are 105°C rated which is good to extend service life. And all of these are interchangeable and would be used in the same types of roles.