Does preferment add sourness to taste of bread?

Your question has a couple layers, so I'll peel those away one at a time!

What causes sourness? Sourness (the taste, speaking generally) is caused by acidity. The acid (i.e., sourness) in sourdough is created by bacteria (e.g., Lactobacillus spp. which are generally benign and sometimes beneficial, such as inhibiting the growth of some harmful bacteria). More bacteria makes more acid, and more time permits bacteria to make more of the acid, and to multiply. Hence, more time (generally) makes more bacteria and more acid.

Fundamentally, pre-ferment does add sourness to the taste bread, for the aforementioned reason: the extra fermentation time (of the pre-ferment portion of the dough) permits additional time for bacteria to cultivate and produce acid.

The other part of your question makes a distinction between time: The time duration of pre-fermentation does have an effect on sourness. This is for the same reason: More time permits more acids to be produced by the critters in the dough, which makes the bread more sour. This is supported by your experience: What you're calling "preferment bread" has a longer first (pre-)fermentation stage, and you report it as more sour; in contrast, your "sponge bread" has a shorter pre-fermentation, and you report it as less sour.

I'm using some hedge-words ("generally", "basically", ...) because there's undoubtedly a lot more happening with interactions of starches, acids, enzymes, bacteria, yeast, etc. than I'm discussing here.

I hope it helps! Have fun with your sourdough. Let me know if I've missed the point, or missed part of your question.

More musings below the cut...

Your question (and associated dialog) got me to think about the connotations of terms. This glossary includes some definitions and examples of terms, and how certain types of pre-ferment (e.g., biga, poolish, sponge, pre-ferment) have subtly different connotations. But basically a pre-ferment by any name consists of flour, water, and some source of yeast: either a previous pre-ferment, commercial yeast (e.g., active dry), or some other source (e.g., grape skin).

Other SA questions for additional reading... This question discusses sponges and pre-ferments as pertains to sourdough. This question discusses the soaker as a non-yeast-containing portion of dough, which permits more autolysis ("self-digestion" by enzymes that exist in the flour/grain itself) without necessarily much fermentation.

Other sourness musings... Even straight dough ("single stage" mixing all ingredients together at one time; i.e., without using any pre-ferment technique) can become (desirably) sour; using less yeast and a correspondingly longer fermentation time will produce a similar (albeit less pronounced) effect. Separately, actual acid could be added directly to the dough, as suggested by this KAF recipe, but that's not necessary or common.

I would add a slight clarification to hoc_age's great answer, since the question mentions using "only yeast" and does not mention sourdough explicitly.

Assuming the question is referring to commercial yeast, a long pre-ferment can still result in some sourness in the flavor. However, unlike sourdough, the types of bacteria and acids produced may not be as consistent. Sourdough cultures are selective environments that only grow certain types of bacteria, like certain Lactobacillus strains, which can survive in an acidic environment.

Flour and water mixed with commercial yeast is generally not a particularly acidic environment, so all sorts of things will grow -- and your dough will resemble something like the early stages of a brand-new sourdough starter, with a variety of naturally occurring yeasts and bacteria, many of which live naturally on flour or come from air or water. Eventually the waste products from the yeast and bacteria will create acids.

In comments, the question of acetic vs. lactic acid came up. The quantity of acid produced by yeast is not appreciable. However, bacterial fermentation will produce both acids (and a few other minor ones in very small quantities). In general, the ratio of these two acids depends on a number of conditions as well as the ingredients. Fermenting at higher temperatures and with a wetter pre-ferment (e.g., a sponge) will increase the lactic acid production compared to acetic acid, while dry-dough fermentation at lower temperatures will often have a stronger acetic acid taste. Note that acetic acid is more volatile and thus gives a stronger taste and smell, so not very much of it is required; it also has greater mold-inhibiting properties than lactic acid. (This is probably the original association of particularly "sour" dough with prospectors in cold temperatures; in those conditions, the bread produced could taste much more sour.)

So it's not only time but temperature and hydration (i.e., "wetness") of the pre-ferment which will effect how much sourness and the type of sour flavors produced. (In sourdough, the initial amount of starter used in the pre-ferment will also be significant.)

The question asks how to avoid excessive sourness. Aside from the advice mentioned above -- especially pre-fermenting at warmer temperatures and with higher hydration -- one can also just use a soaker instead (i.e., water and flour, and perhaps other ingredients -- particularly grains -- without yeast). If the goal is to maximize other flavors beyond sourness, a soaker will allow enzymes to break down the grains and release flavors, but acidity will be less without the yeast. Another possibility is to use a "mash," which is effectively a soaker heated up somewhat to speed up and maximize enzyme activity. Sometimes various dough enhancers (e.g., malt powder) can also be used instead or in addition to speed up flavor development.