Ah, yes. As I suspected, the mash and kettle pH are probably a little lower than desirable due to the lack of alkalinity. That extra tenth or two of pH depression can make the difference in the hop expression. As we all know, hops have alpha and beta ACIDS in them. Because of the acid dissociation effect (pKa), the greater the difference in pH between the reactants, the more likely they are to dissociate and react. In the case of wort, if it is more acidic, its less likely to react (extract) those alpha acids. That is the likely reason behind the muted hop expression and bittering.
Since Steve has the supporter's version of Bru'n Water, he has the ability to use baking soda effectively to provide alkalinity in the mash and assess the total sodium it delivers to the overall wort. As I've preached, alkalinity is not added to sparging water since elevated alkalinity is one of the factors that increases tannin extraction during sparging. Therefore when using baking soda, the concentration in the final wort is diluted when the low alkalinity sparging water is added into the system. The supporter's version includes that calculation and it predicts what the overall concentration of Mg, Na, SO4, and Cl are in the final wort due to these unequal mash and sparging concentrations. (The Ca and HCO3 concentrations are only calculated for the mashing water since there are a variety of reactions that alter their concentrations for the final wort.)
Baking soda is readily available and soluble and suitable for brewing usage. In the mash, you can boost the alkalinity by about 100 ppm as CaCO3 while only adding 40 ppm sodium. Depending on the dilution from sparging, the net Na concentration in the kettle is likely to be much less than that. Since there are virtually no beers that need more than 150 ppm alkalinity for their mash, its pretty easy to see that the penalty from adding baking soda to the mash could be quite modest.
Say you needed that 150 ppm alkalinity and had to add baking soda that increased the mash's Na content to 60 ppm. With sparging dilution, that could easily be half of that in the kettle. Not too bad.
Brewers are far too dismissive of adding sodium to their wort. Fears of saltiness and harshness abound. I can assure you that modest sodium levels in wort are BENEFICIAL to beer flavor. If you review the large list of water profiles in Bru'n Water, you may notice that the Na level in many desirable brewing waters are in the 40 to 50 ppm range. In my opinion, allowing (or even promoting) that much sodium in brewing water is OK. During our research for the Water book, Palmer did a set of taste tests with a finished beer. Even when the Na level was at 100 ppm, he felt the flavor was improved in some cases. As I recall, AJ, Colin, and I did finally recommend that a more modest limit for Na be included in the book. My reasoning was that we needed to keep the Na level more moderate since it can create antagonistic flavor effects with sulfate and chloride. Bru'n Water users will note that I have included sodium in many of the color or style based profiles. In general, Na is at 25 ppm or less. I do find that it aids beer flavor. Anyone who salts their watermelon or tomatoes will know that sweetening effect of moderate table salt.
So, don't be afraid of sodium and be sure to become a Bru'n Water supporter (shameless plug) if you want to use baking soda in your brewing practice.