For this article a few practical points about both these amps, some of their features and how both amps can sound similar on clean settings. N.B: My comments about the Marshall are only for the Hi Gain Dual Reverb series, as there are other models in the JCM 900 series and their sound and  their design is somewhat different! The amp is set-up according to original specs and equipped with the 5881 valves

When you are reading this article you are probably well aware that Jim Marshall based his early designs for the Marshall amp on the Fender Bassman. Both the Fender Bassman and the Marshall amps have changed throughout the years, but some of their basic features remained the same for quite some time.

The early Fender Bassman came originally as a combo: Speaker and amp all in one box, later versions of the Bassman came as a head with a seperate speaker cabinet. The early version (and the reissue!) came with one channel but did have various inputs for normal- and bright use. Later Bassman versions did have this feature combined in two channels: A basschannel, which creates a more grainy sound, and a normal channel for a brighter guitarsound.

As with both Fender and Marshall, most of their amps carry the same sound. The different models tend to vary in some features. The normal channel on the Bassman 135 can give you a very similar sound as the Twin Reverb. Of course the Bassman lacks reverb and vibrato, but as for its basic sound, it is very identical.

The Bassman 135,  similar as earlier versions of this amp, does have two inputs for each channel: A high impedance and a low impedance. Basically these inputs can be used to bring down your guitarvolume: When playing in a small room you can use input two, when you are doing gigs at larger venues, input one may be the one your prefer.
The Bassman 135 was not designed to give you distortion, it is a clean sounding amp, even when hard pushed, especially  when using the original 6L6 valves.

The Marshall JCM 900 dual reverb came both as a head and a combo version. The headversion of this amp uses only one input. At the back there is a switch for Hi/Low sound. This switch does act in a simalar way as both the hi and low input on the Bassman 135: Set it to low and it will reduce your volume.
The Marshall does have a proper distortion and clean channel, but both channels are very flexible. Channel A will be in most cases used for clean sounds. To get distortion from this channel you will need to put the preamp at higher values (especially when using single coil guitars!) while channel B will create distorted sounds  at much lower preamp values.
To get clean from both channels: Set the preamp values at a low setting and work with the master volume. When you use the amp in this fashion you will find that Channel A can sound somewhat similar as the normal channel on the Bassman 135 (Brigh and trebly), use Channel B on the Marshall for a clean setting, and you will get the more grainy sound from the basschannel from the Fender Bassman.
The distortion the Marshall can create goes from dark, think  60s Plexi, to more trebly modern sound of the 90s.
The Bassman will not give you any of those distorted tones, you will need to use pedals to get similar sounds.

Although the JCM 900 series were around at the late 80s early 90s, the basic sound of the earlier Bassmans could still be found on these amps.

When reading this you may become interested in reissues of certain Marshall and Fender Models, you may even become interested in the various versions both Marshall and Fender made throughout the years, of their basic ampmodels.
What you will find is that both companies produced one basic sound, and they made some variations with this sound. The better you are aware of that basic sound (through recordings of different artists using any of those basic amp designs) the better you are to judge for yourself the differences between the various models and the newer reissues. Be aware that later amps used features requested by  guitar players, features such as reverb, channel switching, master volumes, switches to bring down volume level etc. Sometimes these features are considered as a flaw against the basic design. Maybe you can look at these features as guitarplayers modding their own amps for their own specific needs.
These days we have enough tools to get around some of those issues without modding the amp: You can turn a one channel amp into a two channel amp by using various pedals which will give you different kind of volume- and brightness levels without altering the basic sound of the amp.

Doing reseach on a particular ampmodel is a journey in itself and it can be a eyeopener and a lot of fun too.
Hope you enjoyed reading this article. 
Happy playing and hope to see you soon again,
Eddie