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I have to say that that is the FIRST TV set I've seen that had a signal power meter like that, a real one!. Too bad its a seven year old model, otherwise, it could become very popular with OTA folks.
There are three particular numbers on that screen that are useful:
1) SNR. ATSV requres a bare minimum of 15.2 dB while 20 dB provides some fade margin. Your screen is reporting it's a bit shy on that station.
2) Signal power. ATSC requires a minimum of around -83 to -85 dBm signal power. Your reading is right on the edge but, with insufficient SNR, it still doesn't work.
3) Uncorrected errors. Ideally, this would be zero when receivable signals are present.
Your task is improve both SNR and signal power. SNR is best improved by getting a stronger signal at the antenna terminals. This can be achieved by installing the antenna in its most favorable available location so that more of the desirable signals can be gathered by the antenna. Moving to a higher gain antenna would also be expected to improve the SNR.
Signal power, after first improving your SNR, can be helped by a modest pre-amp, one selected so as to minimize noise figure, with good overload resistance, and an enough gain to overcome all the downstream insertion losses plus a modest margin. You already have one of those.
Something doesn't compute in your setup and numbers. TVF predicts a signal power of -55 dBm and the 751 has a few dB of real gain on UHF. Let's assume a signal power at the antenna terminal of -50 dBm. The RCA should have around 22 dB of gain and a noise figure around 3 dB on channel 40 according to a sample I tested. Assume 50' of RG6 (-3 dB) and -8 dB insertion loss for a four port splitter. That means that the signal power level at the tuner should be around -40 dBm. You're more than 40 dB short, a very, very, very considerable amount.
That means one of several things: Either the signal power at the antenna isn't as predicted due to obstacles in front of the antenna or you have some excessive losses in the rest of the system.
Are there trees or any buildings in the line of sight to WILM?
Have you verified that the amp actually works?
Have you runs a single, know good length of coax straight from the antenna to that Toshiba?
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