First, some mechanics you should consider.
Never send a cold-letter longer than one page, because as soon as a prospect realizes it's a solicitation his/her internal voice will say, "I don't have time to read all this" and that will trigger what I call the "crumple-and-toss response."
Even on one page, you should have no more than three or four short paragraphs. Or, have an opening paragraph of about three lines, followed by a few bullet points, followed by a short closing paragraph.
Be certain to have your letter copyedited by a professional, because you don't want your first impression to include mistakes. A pro should be able to copyedit two or three one-page letters in an hour, making it well worth the $30 to $50 bucks. (If you don't know a copyeditor, contact me at firstname.lastname@example.org and I'll hook you up.)
Test-drive two or three versions of your letter at once, sending each version to a separate group of 100 or so prospects. Whichever letter gets the best response should then be sent to the remainder of your prospect database.
Any response rate greater than 1 percent should be considered very good, because of the volume of direct mail that is sent. So even a great letter will net you probably only 2-3 percent. Is that enough return on your investment to warrant the effort and postage? If not, find another type of marketing activity.
There are two great books on cold-letter writing. "The Power To Get In," by Michael Boyland, and "Selling To VITO," by Anthony Parinello. ("VITO" stands for very important top officer.) I'd recommend you buy and read both books and then meld their two styles into your own model of letter writing.
Plan out a four- or five-step drip process before you ever send your first letter. Most prospects won't buy the first, second, third, etc. time, so you must prepare to contact them multiple times. If you prepare this in advance, you'll often find that ideas you have in the fourth or fifth steps will allow you to go back and improve the letter for step one.
Second, marketing effectiveness. The best results I've ever gotten from a one-step campaign came once I started using the eight-second rule. Basically, this rule states that you have only eight seconds to grab the prospect's attention before he/she makes the first crumple-and-toss decision. So your opening sentence MUST be compelling enough to overcome that impulse. And by applying that rule twice (figuring you have eight more seconds to get the prospect to commit to reading the whole letter) you'll get the best result you can.
What is compelling to prospects, you ask? Lots of things, I'm sure, but when it comes to evaluating cold-letters it's only one -- results that are important to them.
Before you put one word on paper, you must know what results your prospects REALLY want when buying what you sell.
You mentioned high quality. If you say "high quality" to anyone his/her "yeah-but" response will always be "high price." What good is high quality to someone trying to live within a budget imposed by someone else? (I'm not saying high quality isn't great, but you must learn whether the majority of your prospects want that before you put it in a cold-letter.)
Find out what your prospects really want by asking your current customers why they bought and how what they bought impacted them. Learn their wording for these results and put that wording into the opening sentence and/or bullet points of your letter. (FYI: That's easy to say, but VERY hard to do.)
Always include a postscript "P.S. Xxx ..." after your name, because studies have shown that the vast majority of people who open a letter from an unknown party will quickly scan to the bottom to answer the "who sent me this" question. Studies have also shown that when this happens, if you include a postscript it will actually be the first thing that gets read. This gives you an advantage, because you can put something in the postscript that creates curiosity without really discussing what you sell. I once sent a cold-letter to one targeted company that represented a huge cash-cow client for me. In that postscript I wrote: "P.S. I realize that only two occurrences is not statistically significant, but since both made the same mistake I thought you might want to take action."
Basically, I had "mystery shopped" the prospect's business to see how effective their salespeople were (I'm a sales consultant, trainer, etc.). I sent the letter on a Wednesday and had an appointment with their VP of sales the following Tuesday. One letter, one appointment. (FYI: That's never happened for me before or since.)
Last, if you have it available, work in some sort of testimonial from a current client. ("Selling To VITO" has great ideas along those lines.") Executives want to make every decision quickly and easily, and knowing that other executives have already bought from you and love your stuff helps them do that.
I hope this helps.