The phenomenon you’re inquiring about is known as the “occlusion effect.” If you Google the term you’ll find some good articles on the subject that will go into as much or as little detail as you’d prefer.
From a clinician’s perspective, your audiogram as well as the size and shape of your ear canals will dictate what type of dome (assuming we’re using an open-fit hearing aid) we would select to help you achieve the best results.
It’s perfectly acceptable for you to ask the professional you’re working with why they selected a particular dome style for you and to inquire if you could benefit more from another dome style. But be careful how far you go with this. If you try to start taking the reigns the professional you’re working with may very well let you (whether what you’re asking for makes sense or not). In your case (CostCo) you’re probably dealing with a non-commissioned specialist, which means they’re more likely to take the path of least resistance (making whatever adjustment you ask for, whether beneficial or detrimental) since your ultimate satisfaction (and whether or not you return the hearing aids) won’t affect them financially.
Also, it’s worth noting that there’s more to achieving the low frequency gain than just plugging the domes on. Think of the tulip or double-domes as “enabling” low frequency gain and then programming adjustments determining how much low frequency gain there will be. With your hearing loss I’ll bet I could easily give you more low frequency gain (too “boomy”/“loud”) than you’d care for with simple program adjustments and either tulip or double domes.