I have a marshall dbs 7400 bass head, both the deep and bright switches are faulty. i have purchased new switches and want to know if i need to get a tech person to fit or are they easy to fit and i could do it myself. cheers steve

A silly suggestion is to plug in a different guitar and see if the problem still happens. Have you checked the link your using to connect to the amp? For example is it a faulty lead, a loose jack plug on the end of the lead or an intermittent signal from a wireless connection? You could also check the pickups on the actual bass guitar.
Pick ups don't often cause issues but they can become faulty because of age, corrosion, short circuits and if they're badly fitted.

You would be Seamus?

Eminence are not a high-priced speaker. You could replace it with one of the bass guitar speakers here, like the large magnet CB15

http://www.usspeaker.com/15.htm
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Hi, what speaker do you have connected directly to the head? What output are you using to go to the PA?

make sure the standby switch is operating correctly.Sounds like the power tube.

Conseguiste el diagrama ?

faulty speaker cable?

It sounds like you bought a defected amp. You should contact Marshall and tell them the problem. You should have some kind of warranty.

It is NOT clear what you are trying to do... HOWEVER if you are attempting to extend a bass potentiometer remotely, you CANNOT use the four wire flat telephone line as it is unshielded and you will pick up hum.

Well i know ive destroyed a guitar amp with a bass guitar before :-p this is why they make bass amps and guitar amps instead of just amps

You are ultimately limited by the available power of the internal amp. You can set controls for bass boost and turn off the treble but there is only so much power available.

HINT: If you are using speakers smaller than 15 inch, that will limit the bass as well. Use larger efficient speakers.