Abbey,
Sorry, I wasn't clear enough on what this amp already has in it. On the back it looks like this
L channel R Channel
Output: 3990 3990 3990 3990
Output: 3990 SJ6451 3990 3990
Driver: SJ6387 SJ6387 SJ6387 SJ6387
Output: 3990 3990 3990 3990
Output: SJ6451 SJ6451 3990 3990
Since this would ostensibly be a studio amp, I'd like both channels to match. If they can simply be matched by adjusting bias and level, I suppose that would be fine. I have doubts though, that these transistors sound the same. Could very well be somebody's friend repaired it, called it good.
I could probably track down more 3990's.
As for the diodes, the following is a conversation about D108 and D109 from the Crown Forums. Guy named Shawn under the name "Raindog Industrial Arts," someone named "DGlass" and Dave Engstrom, Crown Technical Support: http://www.crownaudio.com/forums/index.php?/topic/1482-dc300-recapping/
Shawn:
Replace D108 & D208 germanium diodes as they are the achilles heel, always leading to distortion and coloration when they go bad, which does not take much effort. I'm working on substituting some modern day equivalents as the original spec'd ones are hard to get and the circuit is still weak after you replace them with new ones. The old part number is 1N270. I already used 1N771 but has even less current capability.
1N34A is in my amp right now and it is a "modern" part but it has not been on the scope yet. It does work fine, for now. *beep* bad spot in a design for a light weight germanium diode.
DGlass:
D108 and 208 are on the negative half of the Current Limiter circuit and are basically only on if the amplifier is protecting. They're not even in the direct audio path so it may "brings the science of the 90's to vintage equipment" but it won't improve the signal. 
Shawn:
I think there may be some confusion here; D108 and D208 are in the BIAS SERVO which in turn becomes part of the VI limiter when the VI limiter is in effect. Faulty D108/208's will affect the bias of the output stage of a DC300A. A poor bias setting will certainly generate distortion even though these diodes are not in the feed back loop or in the signal path.
If the output of the DC300A is shorted or driven into a very difficult load for a sustained period, it is very possible the VI limiter circuitry will force too much current through these diodes. The peak operating current of a 1N270 is 325mA @ 25 degrees Celsius!
The Central Semiconductor part CDSH270 is indeed a better part for this application as it has a wider operation temperature and the forward surge current is higher. The Central part also has a continuous forward current capability much higher than the “old school” 1N270.
If you talk to any true professional service tech that worked on Crown equipment in the 70's and 80's, they will most certainly site these two diodes as one of the known problems with DC300’s.
Dave Engstrom:
D108/D208 are part of the current limiting protection circuit, not the bias network. These diodes generally became faulty as a result of the negative current limiter being faulty (shorted). Though a germanium diode would have slightly higher leakage than a silicon device, if Q103/203 was not biased on (excessive output stage current) it wouldn't matter what condition the diode is in.
The 1N34A may be a better part but unless the amplifier is working into a load impedance lower than 2.7 ohms this part of the circuit never turns on. If it is initiated the distortion caused by current limiting will more than exceed any component benefits.
Shawn:
The DC300A VI limiter circuit also once known as SPACE is not transparent to the function of this amplifier when it is "not in use". The VI limit circuitry is an integral portion of this amplifier's output bias, signal output and feedback path. If you disable the VI portion of the schematic, the DC300A can not be biased and it will not function properly. When D108/208 are at fault, the entire amplifier's performance suffers. The VI limit control circuit is directly tied to the bias of the output transistors.
Also, the negative feedback loop is feed back to the DC300 OpAmp right through the middle of the VI limit circuitry.
I think the VI limit portion of this design is being treated like a separate entity when in fact it is embedded deep in the heart of this amplifier's design and function. It is not a separate switch that turns on and off yet it is a feature embedded in the core of the DC300A.
I think I will have to provide some performance data based on a known working DC300A with D108/208 substitutions so you folks can really see the effect the VI limit design has on the performance of this amplifier as a whole.
Unfortunately, this is where the conversation ended.
My line of reasoning was that if there is indeed a more sturdy replacement, it's neither costly nor difficult to do.
