Should I purchase an expensive phono preamp

You might want to look into modifying your RB250. Origin Live modifications to the arm make this piece a category killer that will improve sound defination, character, etc... . There are good sights on the internet where you can read about this. Also look into rewiring for the arm with something like Cardas. Check it out.

EJCPA
A very popular item on A'Gon which sells mopst often within a day is the EAR MM or MM/MC tube unit.Selling new from $1K to $1500 it sells used for $700-850.It allows experimentation with different NOS (New Old Stock) tubes.Another one in the range is the acoustech which won many awards and you have it on your list (none of my thoughts go above this.Other than a few like the $2800 Krell it seesm like the market gives choices up to $1500 than jumpos to $4K and over.At $1500 the Conrad Johnson tube unit is very warm and fine unit.Lower in price spectrum the Lehman Cube for $500 or so.The Graham Slee is an excellently reviewed line as well.But I am being a tube guy I might suiggest between Joilda who just came out with a tube unit at a killer price of $450.Too new for reviews but at price from a company known for good value I don't think you can't lose.I might have bought it but I have a table with an arm that allows for quick change of arm/cartidge so even though I am a tube guy (have an EAR intergated) I am going to get $1K PS Audio.Has flexiblity to change loads easily on back (many including expensive ones require you to open hood to change Dip switches).But also has volume (if you even wanted to go staright into an amp.Also allows for precise input matching geting gain higherst before distortion) as well as mono and phase button on front (not all LP's are made in absolute pahse so on some recordings can make big diference).Think it's a bargain at $1K.But in a hi end system (or any for that matter) the caretidge and phono section effect sound more than turntable and arm.If you step up from a reciever (as good as that one is) your going to have comesurate gear when you do.But in mean time it should sound much beter than the recievers stock unit.Tubes also rolling (changing) and that can be fun to experiment with.But your list of $1500 and less is pretty complete with excellent choices.After 3K hours or when bug bites write back about catridges and sperates or an intergared amp!Great thing about this hoby is you can never stop upgrading but there is a point where the retuirnbs diminish to the point where you don't HAVE to upgrade and can buy gobs of (now) overpriced original LP's on Ebay and get what has become too nutty on CD/SACD/DV-A and what ever new fangled replacement they come out with.
Have fun.
Chazzbo

I would agree with your assessment relating to diminishing returns. Not knowing your budget I would turn my attention to your amplifier and put your money towards improving this side of your system. If you wished to purchase preowned you could pickup a Naim Nait 5 (integrated) and a Rega phonostage which wouldn't be much more than the Acoutech PH1. All depending upon your speakers this would prove to be a good phono match for your Nottingham until you are ready to upgrade to a Spacedeck or beyond. Have a great day...Gerry.

Like nsgarch I feel your reciever is the choke point in your system.You did not mention the type of speakers you are driving or the number.Why not consider selling the Pioneer? adding the amount you would have payed for the AcousTech[my choice out of the phono stages you mentioned] and buy a high quality integrated with a phono stage built in.Then any mods you make to arm or cartridge will be audible improvements instead of being converted to over processed transistor hell.Or you could buy the AcousTech not expect too much and when you upgrade to a line stage integrated be blown away by the improvement.