Hi guys, here is my $0.02:
I personally really like the Tung-Sol or Raytheon 5687's in our preamps. The Amperex 'PQ' 7119 can also be very good, if you want more sparkle and detail, but they can be microphonic.
The Mullard GZ34's are an exceptional choice for rectifier and there are many varieties within this family. Most notable amond them possibly is the metal base GZ34, but $$$!
I am expecting an EML 5U4G to experiment with in various of our designs. The Transporter is the only product that allows drop-in use of the EML tube, at least in terms of physical limiations. Circuit-wise, there is no problem using the EML tube in any of our preamps, but the tube is MUCH too large to fit inside with the lid on. A Greenlee punch is the best way to 'machine' a hole in the lid.
Most machine shops, unless a friend, are not likely to work with you, making a single hole in a single piece. There is the cost of setup, programming, etc.
The MWI cap upgrade is SIGNIFICANT for any of our older preamp designs that did not utilize our own custom capacitors.
The new PS 9.0 power supply with tube rectification and dual-discrete SS voltage regulators is also a SIGNIFICANT upgrade that may be used in our SWL 9.0SE and SWP 9.0SE designs. It is the same design for the external supply for the SWLP also, but I still need to revise the design to allow for powering the unit on from the preamp power switch. The current upgraded PS 9.0 circuit is lacking the relays needed for this, but will be added soon.
Thanks, I hope that this helps!