Arthur, Welcome to the world of 78s via an RIAA phono stage ! I am thrilled you are enjoying the sound. Home HiFi is really about the enjoyment factor. Nobody can take away our enjoyment. However, some folks can possibly help you increase your enjoyment factor.
Since you are using a USB compatible table/preamp, perhaps you can check out the audacityteam website. These folks are at the forefront of analog to digital conversion. Their programs are tailored to use no preamp EQ as they provide the proper curves. Check out their ultimate collection of EQ playback charts and recommendations. They deserve highest accolades for their compilations of playback info for us spinning discs. They cite another site, midimagic as their "go to" reference. Both of these sites are very informative and enlightening.
Now, let's get down to your AT-95 with 78 stylus. Is it a sapphire or diamond ? 2.7 mil or 3 mil conical tip ? 300 hours is a conservative amount for a genuine diamond. 50 hours of spinning/playing time can be a liberal amount for a sapphire. Condition of discs and styli governs real time allowance. Let us dissect 50 hours, shall we ? An average of 3 minutes a side is about 20 sides per hour. 50 x 20 = 1000 sides. That's a lot of music, even for a sapphire tip. I examine my sapphire tips often, under a microscope, and keep my discs clean. I can get a few hundred hours from a sapphire tip if I am very careful about the cleanliness.
Arthur, your original question seems to focus on how to get better sound. RIAA EQ was not "standardized" until the mid-1950s. In spite of the declaration by the RIAA in 1953 and record companies agreeing to conform in 1954, many record cutting engineers, producers and company men held out for various reasons. More important than why is the availability before the RIAA EQ, originally known as RCA's "New Orthophonic" playback curve. Stateside, there were about four "usual" curves in use by 1954. Unfortunately, in Europe, there were a different set of four. Recordings from the UK and Europe which were transcribed for the American market were actually uncertain in their EQ choices depending on vintage ! We can all see how having many playback EQs on hand can be helpful.
Orthoacoustic and NAB EQ curves were established before WW-II. Most EQ curves specify a Turnover and Rolloff frequency and some include what is known as Bass Shelving in order to reduce rumble and woofer pumping. Orthoacoustic and NAB EQ used a 500 Hz turnover. The RIAA curve also specifies 500 Hz. This is the asymptote frequency which is +3 db above the 0 db reference at 1000 Hz. However, Rolloff differs. NAB specifies 1592 Hz(100 uSec time constant) as the (asymptote or changeover) frequency -3 db below the reference of 1KHz. RIAA specifies 2122 Hz (75 uSec) as their EQ rolloff point. So, to simplify, playing an NAB "encoded" disc with an RIAA "decoded" phono playback curve, the RIAA EQ will provide less high frequency cut, less rolloff. Also, with the bass shelving being slightly different, the RIAA playback will most likely have a bit more low bass boost. You see, most playback EQ curves use a 6 db octave slope, boosting freqs below the specified Turnover and cutting freqs above the specified Rolloff.
In the above example, RIAA EQ is not that different from the NAB or later Columbia "LP" EQ. As you mentioned, simply reducing treble a bit increased your enjoyment. Way back then, there were other EQ curves which specified 250, 300 or 400 Hz turnover. Playing a 250 or 300 Hz turnover encoded disc with a 500 Hz turnover decoding playback curve can change the sonic character of some musical instruments. With a dull sounding disc, beginning the upward slope toward the bass boost above the encoded turnover freq, can sound beneficial, since we are boosting some mids which define instrument character.
While many post-1926 and prewar discs were cut with 300 Hz turnovers, some were cut with 500, 630 and even 700 Hz turnovers. The term "house sound" can be applied here, appropriately. In 1951, the AES, Audio Engineering Society, developed a playback curve "averaging" among the many house sound EQ variations. Their reasoning was exemplary. If the record makers could standardize, conforming to producing discs which sounded very good utilizing this "AES" EQ for playback, more discs could be sold. The audio biz could grow and audio companies could prosper. Most importantly, our home HiFi enjoyment factors could be increased which would also sell more hifi gear. The AES EQ curve has become essential in increasing both my 78rpm playback enjoyment as well as my early LP and 45 rpm enjoyment. Too many 1950s record companies persisted conforming to this EQ curve and some even did not disclose that fact !
The AES EQ specifies 400 Hz turnover (398 uS) and 2500 Hz rolloff (63.5 uS). Bass boost shelving was optional for the recording engineers, as long as their discs sounded good using this playback curve. There is another spec which is very important in sorting EQ curves. That is the "db cut at 10KHZ" spec. NAB EQ is -16db at 10KHz. RIAA EQ is -13.7 db at 10KHz. AES EQ is -12 db at 10KHz. Yes, folks, things get confusing. If we play an early 78 or AES type disc with the RIAA EQ, highs will be cut and we will hear this.
Then, there are the discs which may sound best with a 700-800 Hz turnover. There are no inexpensive preamps or equalizers which get all this right. Equalizers, like graphic or parametric types, do not conform to the sloped curves used by the early record makers. Neither do normal bass and treble controls. However, tone controls and equalizers can indeed help increase playback enjoyment factor.
What is best is finding, resto-modding or making a phono preamp which has multiple playback curves easily switched in. The fact is that the small db increment differences among EQ curves are clearly heard and switching in the correct EQ will be easily heard. Quick switching is much easier than fiddling with equalizers, but this comes at a price or electronic building skill level.
For now, keep on enjoying the spinning discs. Everything sounds good by itself. RIAA EQ and tone controls is certainly going to sound better than an acoustical horn equipped electrola phonograph system. Check out the audacity and midimagic sites to put this info in perspective...