Pre-Amp vs. Integrated...why?

I have been a vinyl listener now for over a year and one concept that escapes me is the "need" for a vinyl pre-amp.

I understand the need if your current amp does not have a Phono input, but if you have an amp that has a phono input, why buy a phono pre-amp to run your TT through?

Is it standard thinking that the integrated Phono input is always substandard to a good phono pre + amp setup?

Are there lines that are drawn so to speak, maybe from a dollar standpoint, that clearly mark the quality in phono stage? (i.e., are cheap $50 pre-amps worse than a $300-$400 integrated or vice versa is a $400 integrated usually going to get crushed by the $500 pre + Amp setup?)

Are there exceptions where a lower cost integrated can hold it's own with Pre and Amp setup?

I am in the process of assembling a second Vinyl listening station in my house and want to start from scratch with new and different equipment. Something different from what I have had over the past year.

Can anybody clarify this or demystify this for me?

My current setup:

-LP120 turntable / AT440MLb cart
-Marantz 2220B Amp
-Infinity RS2000 bookshelf speakers

My future/second setup:

-TT? (I have ideas)
-Elac B6 Debut bookshelf speakers (already have)

Room specs are about 12x12 and this is for vinyl listening only. Other stuff is a bonus but not necessary.
Your Marantz is a receiver having a tuner, phonostage (sometimes called a phono preamp), a linestage preamp and an amplifier. There are integrated amplifiers having a line stage preamp and an amplifier combined. Some also include an internal phonostage. Many prefer a linestage integrated with a separate phonostage for cartridge matching flexibility and sound quality, however this is more true at a budget higher than that under discussion here.

In your specific case I would recommend the purchase of an integrated amplifier or receiver which includes internal phonostage. Check out the  A-Sxxx series as one example. 
Internal phono stages typically only accommodate moving magnet cartridges, which is what you have.  There are no options for "loading" the cartridge or changing the gain for low output moving coil cartridges.  Some higher end integrated amps will have some adjustments.  

A good phono preamp will give you a wide range of options for loading, multiple gain settings, and overall much better sound quality than the one included in your Marantz.  

With the turntable and cartridge you have now it probably doesn't matter that much, but if you are on a quest to improve the sound quality of your system it will at some point.
agree with mesch...but there are some very low cost phono preamps that would match well with your cartridge, so if you find an integrated without a built in phono that you like...I just bought a refurb Cambridge Solo MM phono pre that sounds great with my Ortofon MM cartridge and vintage Yamaha TT
At the op price point there is not a lot of reasons to go separates. However if you run into a good deal on a stand alone phono box would not hurt to acquire it. But no reason to search for one.
Most low cost ’phono’ installed are just a small section of a few parts or just a inexpensive chip. You get what you pay for.. Generally at the low end of the audiophile journey, your money should be blown on speakers.. (self powered ones... ) mostly.
My 2 cents.
Yes, a separate pre/power amp will beat a receiver for SQ! But the sky's the limit for cost! I, however, like and use vintage gear (have a Marantz 2235) - a large collection! Presently using an RGR 4 preamp, GAS Son of Ampzilla and DCM Time Windows (52779, 52780). Total cost: about $800! SQ to die for! Judicious choices of vintage gear can yield outstanding SQ - at affordable cost!
The OP can get a Bellari tube phono stage for $275 (free shipping) and use it into the AUX input on the 2220. EBay seller is "decibelaudio" in Chicago. Superior SQ over the 2220's phono stage! It uses a single dual-triode 12AX7. So easy to experiment with different brands (GE, Sylvania, Mullard, Telefunken ... ).
The nice thing about the Bellari VP130 is that it is USA-made! I have the Bellari SUT for use with my mc cartridges. A quality product at an affordable price!
Another choice for the OP could be the NAD 3020 V2 integrated amp (phono stage, line stage, DAC, Bluetooth-ready). 40 wpc (twice that of the 2220) and pre-out jacks to drive a more powerful amp. Available from many EBay sellers for $400-500. I'd take it any day over the 2220! Time marches on!
Just to clarify you understand what the phono preamp is doing right?  its main job is to decode the RIAA curve and to amplify the very small signal from the cart so your preamp can use it. (preamp regardless if integrated or not). The decoding is really the main job with out that you can't play a record. then your into what gain and loading you need for the type of cart you have.
J, the above explanation nails it for cartridge decoding. Like you I spin vinyl for now and really enjoy the combination of Project’s Tube box 2 with a Primaluna Dialogue HP with 8  kt150s driving ML Ethos speakers.
Have fun with your search!
The Parasound JC3+ is a great phono preamp I use it with my VPI prime turntable, I started with a Schiit mani phono pre but the JC3= just blows it away, it's not cheap $2,950.00 but it works great and it is for both MM and MC cartridges.The Schiit mani is only $129.00 but it's good to start out with.
Got a lounge phono preamp in to a Sansui tube receiver. I did this because the phono pre on the sui was garbage. It now sounds great. The lounge cost me 300$. 
There are many reasons for using a preamp and amplifier(s) versus an integrated amplifier or receiver.

In my case, my speakers (Maggie 3.6) need significant power, more than available with a receiver.  I am running Mac MC501 mono blocks that make them sing.

I am looking into getting back into vinyl and my tube preamp accomodates that if I make that decision.

Sometimes other components make the choice for you.

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