Substitute for dashi

Sure, it's really not that difficult if you've actually tasted several kinds of dashi-jiru. It's more a matter of experience. There is decidedly a flavor to each category of dashi; it's not just "umami" or you would be able to get away with just throwing in a bunch of MSG into a bowl of water. But the flavor is mostly from aroma, like with other types of soup stock, since you haven't added any of the basic "tastes" other than the kombu and occasionally residual salt from the dried fish at the point the dashi is made.

At home, I often make a vegetarian one with kelp and porcini, sometimes with added cabbage; this the closest I've been able to come to the katsuo-dashi taste without actually using fish ingredients. It's a variation of a more standard Japanese kombu-dashi that's made with kombu and sometimes dried shiitake, but works for a broader range of dishes than the shiitake or minimalist kombu-dashi.

In practice, the main categories of dashi are these:

• Katsuo-dashi: Made with dried, cured skipjack tuna (aka bonito), and shaved. It has a slightly bitter taste. It is best made with the addition dried kombu. Tastes richer with either thicker shavings of katsuo-bushi or just-before-cooking shaving, when possible.
• Niboshi-dashi: Made with small dried whole fish (sometimes with heads removed, sometimes not, depending on the type of fish). Generally has a more pungent, richer flavor. There are several potential kinds of fish used and there was once substantial regional variation in this category, so the taste can vary quite a bit depending on the exact fish variety and how the fish was cured.
• Kombu-dashi: Made just with kombu, usually for adding a little bit of aroma and complexity to simple dishes, but rarely for soups, except for nabemono, where you'd typically have additional seafood or meats in the hot-pot. (I usually lump kombu-shiitake or kombu-porcini dashi in this category, but for no particular reason).
• Tori-dashi or gara-dashi: Made with chicken or other fowl bones. Except for fact that mirepoix isn't usually in the Japanese equivalent of this, so you won't have the celery base, it's similar to a typical Western-style chicken broth.
• Ton-kotsu-dashi, made with pork bones. Generally pretty hearty and full of fat.

Dashi is the analog to a soup stock, so it generally does not have added salt. Accordingly, you won't serve a straight dashi with an udon; you'd turn it into a broth, generally called "kakejiru". This will contain added salt, shoyu, and usually some combination of sugar, sake and/or mirin.

Before you put salt in it, you'll mostly "smell" a difference. After that, you can certainly at least distinguish between katsuo and kombu dashi, and with a little practice, you can distinguish between niboshi-dashi and katsuo-dashi. For me, the difference between katsuo-dashi and the instant "hon-dashi" is usually not subtle; the hon-dashi is a bit aggressive and harsh to my taste, but not everyone feels that way. (For what it's worth, I'm as close as practical to being a vegetarian for someone who regularly dines out in Japan). Tonkotsu-dashi and tori-dashi are somewhat rarely used in home cooking (though there are some exceptions in west Japan and Okinawa), but the taste difference is pretty obvious.

I actually tend to prefer niboshi-dashi over katsuo-dashi, but it depends on the type of fish. There's a dried sardine from Korea that I find a little harder to stomach when used in dashi.

I can't speak to allergies, but I suppose it's possible that different types of fish may cause some people to react differently. I suppose you'd have to ask an allergist.

Dashi is almost never used without adding additional seasoning, generally some combination of mirin and/or sake, salt, and soy sauce or miso, and often a small amount of sugar. If your flavor isn't strong enough, most likely your culprit is inadequate salt.

I never make dashi without using konbu, but I'm vegetarian, so mine is typically based on konbu and dried porcini, or just konbu alone for certain applications. I'd probably only consider skipping konbu if I was making a dashi based on niboshi, also called iriko (dried sardines), as is common in Kyushu and some other regions. Iriko are pretty aromatic and apparently contain enough glutamates that at least some people skip konbu.

It's not unheard of to leave out konbu in katsuodashi, but unless you're using thick or freshly shaved katsuobushi, you'll end up with a basically bitter, uncomplex version that's mostly suitable for use in dishes with lots of ingredients. I might use a konbu-less dashi in a dish that has cabbage in it, because cabbage provides its own source of glutamate proteins.

Dashi is the analog of stock, not the analog of broth. Stock typically contains minimal to no salt, and therefore the primary contribution is aroma, which is only perceived as flavor once you add something salty to it. Once you add seasonings, dashi becomes "kakejiru" if it's at the right saltiness level for noodles. If you make it saltier it's a "tsuyu" or dipping sauce.

If you're going to cook Japanese food, you'll need to get over any issues you have with salt. Actually, if you're going to be making any kind of soups, I'd say the same thing regardless of cuisine. But Japanese food uses salt, or salty ingredients like miso and soy sauce, in relatively large quantities.

Usually, your main dish won't be incredibly salty (except sometimes at breakfast), but most of the sides and soups are fairly high in salt content. The source of the salt varies, as there are two common salty ingredients (miso and soy sauce), and regional things like shottsuru (Akita's fish sauce); wet sea salt is used to adjust seasoning, even when fermented salty ingredients are present in a dish. Most meals include some fairly salty pickled vegetables; the rice balances out the rest of the salt.

One of my Japanese friends lamented that her husband's salt-sensitive hypertension was a big problem for her because she can cook low fat, or low salt, but not both, and low-salt was a much bigger problem for Japanese cuisine, as even high-fat dishes tend to have a fair amount of salt.