If its a used B7 from like a year ago, I would recommend the Kr242 (though costly).
I think Mike Lavigne at WBF has a pair for sale. They dont work well in a standard GG, as the voltage output is too high, but they are heaven in a B7...I know, i tried it myself...
For recti try these cheap ones.
Haltron/Zaetrix/Fivre double mica (ceramic spacer) 5r4gy, Brimar 5z4, RCA/Jan world war 2 (pre 1946) double upward sloping getters 5r4gy. They cost about $100 on EBay. Date code for 1944 is "H".
More expensive is the Philips metal base GZ34 and the Brimar 5r4gy FE code.
If cost no object, then the Elrog 300b or the vintage WE 300b with the Takatsuki 274b recti.
Also the Kron Px4/45/300b are mid high priced but have good reps.
The Lampi EML 45 mesh globe is terrific but sold out for now.
The Psvane "UK design" 300B is very good and costs about $350 a pair on EBay.
I have not used all these tubes, but in severalnthreads people make comparisons and certain popular views emerge.
One or 2 people swear that in general Kron beats EML, and that Elrog/Takat beat all except possibly for the horrendously expensive WE vintage tubes. Takat recti is like $850 and a pair of Elrogs is about $1,200.
Here is a list of tubes and relative prices:
http://lampizator.eu/Fikus/DHT_TUBES.html
EML 5U4g has been equalled/beaten according to many pundits.The KR PX4 seems very popular, would be nice if someone that had tried them and compared to EML 45's could chime in.
As for the rectifier don't have a clue, some say the 5u4g from EML cannot be beaten, some say go full nos with either U52 GEC or Marconi 5u4g.
Try the NUs with a different recti...maybe any GZ34.Iam comparing the National Union 45's with my Psvane hifi 101d and I hate the very forward presentation of the Nation Unions compared to the 101d's being placed in the middle not so much in the front.
Also the vocals are very thick with the National Union 45.
I have ordered the Sylvania 5u4g and 5x4g with National Union 45.
Currently stock is 101d Psvane white base and 5c4u.
What should I be trying for best imaging and soundstage.
KR 274b is a much better recti, the 101d is very good for output valves, and much better are KR 242s if you can find them. KR 300bs are also better than the 101dd, and I haven't heard the Elrogs or Takatsukis but they should be too, though mich more expensive.
Sold my EML recti after listening to KR, and haven't tried px4 yet but the 242 takes the big 7 to almost GG levels
5u4g