You will need a phone pre-amp for a couple of reasons. The first reason is that magnetic phono cartridges put out only a very tiny signal, and must have considerable Voltage amplification applied before the signal is at an appropriate level to feed a line-level input.
The second reason has to do with the RIAA equalization used when the record was cut in the first place. The recording industry settled on a standard equalization some decades ago for the sake of getting more music on a given size of record. Using no equalization would have resulted in very few selections on one LP, because bass frequencies take up a lot of real estate. So, a standard EQ was established that reduced the bass frequencies relative to higher frequencies, allowing the grooves to be much more tightly spaced next to one another.
The other part of the standard provides for correction of the recorded EQ during playback. To meet the standard, the phono preamp must have built-in equalization that is essentially just the opposite of the EQ used when the record is first cut. To over simplify, think of turning the "bass" control down when the recording is made then back up when the recording is played back. This is essentially what the RIAA equalization standard does.
So, in order to play back a record with RIAA equalization, you need something that is designed for the purpose. The SEX (and other amps with line level inputs) do not have that facility built-in and a phono preamp is necessary.
If you want to try a simple experiment, plug the output of your table into a "tape", "line", or "aux" input on your current rig. You should find that not only is the volume greatly reduced, but the bass will be pretty much non-existent.
BTW, during the hey-day of Disco in the 70's, club DJs had access to "disco cuts" of the hits of the day. These consisted of a 12" record with only one song cut on each side, to allow for more room to record the bass characteristic of the genre. Prior even to that, radio stations, particularly if they played classical and/or orchestral music, often had records much larger than the 12" diameter, again with the idea that more real estate allowed for a higher fidelity recording. So, there have been several "standards" over the years. But, the standard for home use has been the RIAA since, I believe, the 1940s.
So, yes, you will need a Seduction or an Eros, or some other kind of phone preamp to go with your SEX. To get started with, though, if your current rig has a "line out" or "tape out", you could use that to feed the SEX amp. This would allow you to use the "phono" preamp you already have to play records while you decide.
