Marc, I have spent a considerable amount of time rolling the 6550 power regulator in an Ref 5se ( more than any sane person should) whereby one may tailor the signal to ones own taste, varying from the Anylitical/Linear to the Harmicaly Rich.
The way to facilitate this tonal shaping is by the use of disparate NOS 6550's manufactured in the main by Tung sol from the late 50's through to 70's and with GEneral Electric 6550A from the 70's.
For my part I would rate the following in tearms of the most A/L through to the most H/R and are entirely my own subjective considerations.
GE 6550A ( Perhaps closest to stock Sovtec ARC 'voicing' however with less grain and more micro detail resolution , a tad smoother when pushed and improved Dynamics.
Mid to late production Tung Sol Three hole Grey Plate ( more similarities than not to the GE's with a touch more Harmonic emphasis )
Early 1960/61 production Solid Grey Plate ( my personal favourite exhibiting a nice balance between the Linear and the Harmonic )
late 50's/1960 Solid Black Plate ( perhaps a little less composed at the frequency extremes when pushed, yet imparts a depth and weight of Harmonic tone and texture, particularly effective on Vocals, think Ella or the Chaiman Of The Board here)
Now the Caviats, I would strongly advise that anyone considering Taking their Ref 5se for a spin to only utilize Thoroughly Tested valves measuring at least 90% emitions and certified safe for Gas and Shorts, preferably sourced from a Recognised dealer or trusted individual. In the main, in order to ensure the safety of your equipment, yet also to ensure the integrity of the valves purchased.
Should your Ref 5se or for that matter Ref 10 ( both 6550's should be closely matched in the latter) still remain under ARC warranty you may consider obtaining sanction from your dealer, however as ARC do not retail NOS 6550's you are most likely to receive the stock * Do not recomend , does not compute , exterminate , exterminate* response!
Although sourcing such valves is getting both more expensive and harder these days it is nothing compared to the Snake Pit that is The Genuine production 6n30 DR, and I would concur with the advice already imparted , however should the intrepid Ref 3/5 owner wish to give it a shot, without selling a vital organ, I would recommend at least a single DR in the associated PS section, adjacent to the Regulator.
This guide may help folks to avoide falling into that snake pit I was on about.
http://www.aca.gr/index/hiend/hiendArticles?row=1988
Jasper.