I use an all tube amp (VOX AC4TV). I have been using a multi FX processor (Tonelab LE), but now I want to use analog effect pedals with this amp.

1- Since this is a vintage style amp, there is no FX loop option. I know that as a general rule, time based effects (delay, reverb) are placed after the preamp by using FX loop. What can I do about this problem? Is it a big sound difference placing the delay-reverb effect before or after preamp?

2- And if I use the multi FX processor.. The processor has a preamp inside, and when I connect it to the amp, there is one more preamp from the amp on the signal path.. So actually we are passing the signal through two preamps.. Isn't it bad for the sound and the tone? Is it possible to use MultiFX processors just as stompbox modelers without preamp-poweramp models?

Thanks for your help..
I think the latter depends on a concrete FX processor. Basically, you don't want tp pass the signal through two preamps.

You can put delays in front of the amp as long as you play clean. If you use overdriven sound on the amp - the delay won't work in a conventional way. It's a cool effect though if used right

You can try and set up your effects around a mic preamp/small mixer and mic your amp as usual. So the sound passes a complete amplification chain including the speaker and the its picked up by a mic and is sent to the effects. Its kinda pricy, but the result is absolutely awesome. Guitar is being mic'd all the time anyway.
Try to keep the amp as clean as possible and you'll have no problem. Using two preamps isn't necessarily a problem. However I've noticed that using amp models in a multifx plugged into the front of an amp can sound bad if you don't really work hard to find the right settings. Based on my experience I recommend bypassing the amp/cab models at first and use stompboxes. Then try amp models and see how it goes. It's all about experimenting.