Phono preamps definitely sound ok, good, great, and I advise having the option of return for any phono preamp. However, what you are describing/intending is far from normal.
I agree, something else is wrong, additional eq after RIAA phono eq should NOT be needed, especially for all LP's.
RIAA phono equalization 'curve' grew into the standard for the LP recording/playback industry. ALL modern LP's are cut using reduced bass and boosted highs as per the RIAA spec. ALL modern Phono preamps boost the bass and cut the highs as per the RIAA spec. Minor, very minor differences result, by random parts deviation, or by designer's intentions, repeat very minor.
Prior to that, various recording/playback equalization 'curves' existed. Some phono stages, like the Sunvalley you mentioned, offer the RIAA standard, AND optional curves, intended for playing ancient lps recorded with those historically optional eq curves. Yes, you can use them with modern LP's but that is not their intention. This exists for Reel to Reel tapes also, early eq curves until a standard emerged.
LP, ' long playing' records, getting more content on a disk, was made possible by cutting the bass during recording (thus physically smaller width grooves were needed), and, like Dolby later, boosting the highs helped to reduce noise on playback.
Is your cartridge MM or MC?
Perhaps your Phono Eq has internal optional switches, resistors, settings for various cartridges, and current settings are 'bad' for your cartridge.
I would check with the manufacturer, and, go to eBay or Amazon, pick something you can return, try it, all should be well. If not you have problems down the line.
