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.